PAGES Magazine articles

INSTANT-SOAS workshop, Trieste, Italy, 11 September 2023

Context

Understanding the exchange of heat and mass between the Southern Ocean (SO) and Antarctica facilitated by dynamicallydriven currents, such as Antarctic Circumpolar Current (ACC), is critical for constraining the stability of Antarctic Ice Sheet (AIS) (Martinson 2012) (Fig. 1). As the instrumental records that would inform ice-ocean-atmosphere interactions are minimal, the Southern Ocean-Antarctic Interactions (SOAS) subcommittee, formed in the framework of the SCAR Instabilities and Thresholds in Antarctica (INSTANT) Programme, aims to circumvent this limitation by examining past critical periods, and facilitate the dialogue and collaboration between the proxy and modeling communities. To lay out the objectives and identify the priorities for the subcommittee, the first in-person workshop was held during the SCAR-INSTANT conference on 11 September 2023 in Trieste, Italy. The following sections highlight the objectives of the program, and existing knowledge gaps.

Figure 1: Visualization of the ACC showing modeled modern-surface current velocities with prominent eddies shaping ocean transport ,and exchange among SO and Antarctica. Model: FESOM2 (Finite-volumE Sea ice-Ocean Model, formulated on unstructured mesh, fesom.de) Setup: ROSSBY4.2; Simulations: Dmitry Sein (AWI); Visualization: Nikolay Koldunov (AWI). Image credit: Nikolay Koldunov.

Objectives of the SOAS program

The main science questions that the subcommittee aim to address are as follows: a) What can we learn from new and emerging records of past AIS, ACC and Southern Hemisphere Westerly Wind dynamics?; b) What is the role of changing winds, and surface-, intermediate- and deep-water circulation of the ACC on the growth and decay of the AIS?; c) How do we integrate SO records with those from the Antarctic continent (e.g. from ice cores), and the Southern Hemisphere mid-latitudes (e.g. lakes, moraines)?; d) How do recent findings for the geologic past inform numerical modeling of AIS mass loss, global sea-level rise and thermohaline circulation, and help assess their impacts on global climate?

A major goal is to bring the paleoclimate communities together to decipher past tipping points of climate as the most critical periods where model calibration, and general understanding of involved processes, is required. Such periods of interest include millennial-scale variability, the last glacial termination, the last interglacial, the Mid-Brunhes, Mid-Pleistocene Transition, the Mid-Pliocene warm period, and the Mid-Miocene. Those were periods when the changes in CO2, temperature and sea level were significant – all possible scenarios to constrain the sensitivity of future AIS.

Research and collaboration gaps: Paleo-proxy and modeling perspectives

Accurately modeling ice-atmosphere-ocean interactions in different past climatic states is challenging due to technical and computational limitations, as well as gaps in our process understanding (Colleoni et al. 2018). Experiments from Paleoclimate Modelling Intercomparison Project (PMIP) Phase 4 have highlighted paleo-bathymetry, sub-surface melt rates, sea ice, polynya and bottom-water formation, and the timing and amount of meltwater entering the polar ocean, as some of the major factors that contribute to uncertainty in paleoclimate model simulations (Kageyama et al. 2018).

While the models remain notably sensitive to the choice of meltwater scenario (e.g. Chadwick et al. 2023), sea-ice conditions and paleo-bathymetry, there are only limited constraints on these parameters (e.g. Weber et al. 2014). New proxies, as well as more spatial coverage in the SO for traditional proxies, are essential.

A major challenge for the proxy community is to fill gaps in observational data by increasing the spatial coverage from what currently exists in the International Ocean Discovery Program, and its predecessor programs, as regional variability in the ice-ocean-atmosphere processes can be significant. Furthermore, while marine records provide proxy-based information on processes happening at the ice margin, they do not provide direct observations of inland ice retreat. This knowledge on inland ice retreat ultimately helps quantify sea-level contributions from the various Antarctic catchments that modeling experiments aim to assess for future projections (Patterson et al. 2022). Land-based geological and ice-core drilling efforts are the only means to ground truth-modeling experiments that examine such contributions, and reduce uncertainty surrounding the extent of ice retreat under future warming scenarios.

Improved modeling of the evolution of the Antarctic-SO system would require identifying regions of highest priority to target for traditional and/or new proxy data, which may be better facilitated through the accessibility of paleoclimate model data by the proxy community. The paleoclimate community has identified that accessing model outputs acts as a major limitation to performing data-model intercomparison studies that may facilitate site selection, and proxy development.

Through this seven-year-long program, we plan to provide a timely synthesis of progressive knowledge of SO-Antarctic interactions, and provide a platform to foster collaborations that would minimize the knowledge gaps identified here.

Acknowledgements

We would like to thank the SCAR-INSTANT joint Chief Officers, Florence Colleoni and Tim Naish, and the theme leaders for organizing the conference, as well as the presenters and attendees at the workshop.

affiliationS

1School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand

2Institute for Geosciences, University of Bonn, Germany

3Department of Earth Sciences, Binghamton University, USA

4Alfred Wegener Institute, Helmholtz-Centre for Polar and Marine Research, Bremerhaven, Germany

5GNS Science, Lower Hutt, New Zealand and Antarctic Research Centre, Victoria University of Wellington, New Zealand

contact

Abhijith U. Venugopal: abhijtih.ulayottilvenugopal@canterbury.ac.nz

references

Chadwick M et al. (2023) Paleoceanogr Paleoclimatol 38: e2022PA004600

Colleoni F et al. (2018) Nat Commun 9: 2289

Kageyama M et al. (2018) Geosci Model Dev 11: 1033-1057

Martinson DG (2012) Palaeogeogr Palaeoclimatol Palaeoecol 335: 71-74

Patterson MO et al. (2022) Sci Drill 30: 101-112

Weber ME et al. (2014) Nature 510: 134-138

Current deep uncertainties in the projected global mean sea-level rise result from knowledge gaps in the physical processes involved in the response of the Antarctic Ice Sheet (AIS) to global warming in the coming decades to centuries (Fox-Kemper et al. 2021). Many of the ice shelves fringing Antarctica are at risk of rapid thinning, or collapse, due to oceanic and atmospheric warming, with likely impacts on the position of the ice-sheet grounding line. It is critical for the ice-sheet community to determine whether Antarctica’s margins have already crossed a tipping point, and if so, when and how much mass loss will take place. A multi-disciplinary approach is required to advance the state of knowledge on tipping points spanning a large range of spatio-temporal scales and components of the polar-climate system. This is the overarching objective of the SCAR Instabilities and Thresholds in Antarctica (INSTANT) Scientific Research Programme (Colleoni et al. 2022).

Launched in February 2021, INSTANT held its first in-person conference in Trieste, Italy, in September 2023. The conference had two main objectives: foster multidisciplinary science, and promote and support early-career scientists’ (ECS) engagement in networks. The first objective was achieved via morning plenaries dedicated to each INSTANT theme (Fig. 1): Atmosphere-ocean-AIS interactions from past to future; Earth-AIS interactions; AIS contribution to sea-level projections. Participants were therefore exposed to cutting-edge research across themes, disciplines and techniques, and this continued with open poster sessions. Afternoon community-driven workshops were organized by INSTANT subcommittees and INSTANT partner initiatives (Fig. 1). A mini-symposium on the last day of the conference – Melting Ice and Rising Seas, telling stories and engaging people – enabled interaction with policy-makers, stakeholders and practitioners, as well as indigenous advocates, writers and journalists. The conference demonstrated the need to discuss these important societal matters. Addressing the second objective, ECS led two workshops: one on how to handle a multidisciplinary science career, the second on translating science to policy. The INSTANT Programme thanks its sponsors who funded the attendance of 43 ECS. Many pre-conference and post-conference workshops also took place.

Figure 1: Structure of the SCAR "Instabilities and Thresholds in Antarctica" Research Programme as implemented over the past three years with internal sub-committees (SC). Image credit: Flo Colleoni and Tim Naish.

This was the first in-person conference dedicated to Antarctic research since the pandemic (the last SCAR Open Science Conference was online in 2022). Nearly 300 scientists from 25 countries participated; one third of whom were ECS (shorturl.at/izBCZ). The outcomes from the conference emphasize the urgent need to close the knowledge gap on:

(1) The physics driving ice-sheet grounding line dynamics, subglacial hydrology (and role of geothermal heat flux), and ice-shelf calving. These are all important mechanisms for understanding and computing ice-sheet instabilities and mass loss from Antarctica, but are currently heavily parameterized in models;

2) The physical processes at the interface between the ice sheet and the ocean, which are still poorly known due to the paucity of observations;

(3) Surface-mass balance over the Antarctic Ice Sheet and surface melting, which remain poorly constrained because observations are very localized in space and time;

(4) The impact of glacial-isostatic adjustment on ice-sheet instabilities, which is still poorly understood because our knowledge of the lithosphere and mantle rheology underneath the ice remains limited, and these processes are not always captured in dynamic models;

(5) Ice-sheet instabilities over multi-centennial timescales, to determine whether some Antarctic ice shelves have crossed their tipping point. The timespan of satellite observations indicating a rapid thinning of West Antarctic ice shelves is too short, so paleoclimatic evidence is needed to provide constraints on a longer timescale.

The key themes emerging from this conference are the need to advance science collectively by sharing data and models with the community, and the need to maintain global and regional observational networks (for example to avoid gaps in satellite programs, or the dismantling of geodetic networks). Both require international coordination and sharing of research infrastructures and strengthening of cooperation between the member states of the Antarctic Treaty. The conference was endorsed by the UN Ocean Decade. Recording from the INSTANT Conference is available on the YouTube channel of the SCAR INSTANT Programme.

ACKNOWLEDGEMENTS

We acknowledge the sponsorship of PAGES, PNRA, OGS, WCRP-CLic, GNS Science, the Antarctic Research Center (Univ. Wellington), Canada and Polar Knowledge Canada that supported ECS travel, invited speakers and organization.

affiliationS

contact

Florence Colleoni: fcolleoni@ogs.it

REFERENCES

Colleoni F et al. (2022) Eos: 103

Fox-Kemper B et al. (2021) In: Masson-Delmotte V et al. (Eds) Climate Change 2021: The Physical Science Basis. Cambridge University Press, 1211-1362

 3rd African Dendrochronological Fieldschool, Kitwe, Zambia, 24 July–2 August 2023

Environmental challenges have had a negative impact on African forest resources, which has subsequently adversely affected some ecosystem services that are required for the survival of people. Dendrochronology is a science that helps solve these problems. However, the use of dendrochronology in Africa has been limited due to a lack of experts to support research and training. The African Dendrochronological Fieldschool program was established to develop human capacities in the field of tree-ring science and research. The program was initiated by the Copperbelt University (CBU); through the Copperbelt University Africa Centre of Excellence for Sustainable Mining (CBU-ACESM) in Zambia, to provide basic scientific knowledge and skills to participants in sample collection, preparation, tree-ring measurement, cross-dating, chronology building, and interpretation of results. After completing the training, participants are expected to have gained basic knowledge to help solve various environmental problems.

Training format

The training adopts the North American Dendroecological Fieldweek format. Participants are introduced to various projects on the first day after touring the potential sampling sites. Each participant then chooses the project of their interest. The assigned facilitator takes participants through the project, from the beginning to the end of the training (Speer et al. 2006).

Study focus

During the training, participants were divided into four groups to focus on four different topics in the wet miombo woodlands of Zambia:

i) Dendroecology: Establishment of the arboreal diversity and dynamics of wet miombo woodlands.

ii) Dendroclimate: Determining the effects of precipitation on the growth of Brachystegia longifolia and Julbernardia paniculata.

iii) Dendrochemistry: Evaluation of the metal concentration in Brachystegia longifolia induced by copper mining pollution.

iv) Wood Anatomy: Determining the anatomy of selected tree species from the wet miombo woodlands.

Field sample collection

All samples were collected from the African Explosive Limited (AEL) site in Mufulira District on the Copperbelt Province of Zambia (Fig. 1).

Figure 1: Distribution of sample sites at the AEL site in ßMufulira District, Zambia. The yellow pinpoint icons show the locations with chronologies as reported in the International Tree-Ring Data Bank (ITRDB - notice the lack of chronologies in sub-Saharan Africa). The blue pinpoint icons show the beginning and end of the 40 m transect for the dendroecology study.

Laboratory sample preparation and analysis

After sample collection, tree-ring cores were mounted, sanded and scanned. We measured tree-ring widths in the software application CooRecorder and cross-dated using CDendro (Maxwell and Larsson 2021). We also used COFECHA program (Holmes 1983) to check the dating quality of samples.

Workshop outcome

Organizers and six facilitators from three different countries (USA, UK and Zambia) trained 25 people from 10 countries (Democratic Republic of Congo, Egypt, Ghana, Kenya, Mexico, Mozambique, Namibia, USA, Zambia, and Zimbabwe). Each of the four groups (Dendroecology, Dendroclimate, Dendrochemistry, and Wood Anatomy) that we formed during the training reported interesting results. The Dendroecology group worked on 49 tree species that were sampled in a half hectare plot, and found that the Fabaceae family plants had the highest species richness with 28.5%. This group further found that wet miombo tree species produce annual growth rings responsive to seasonal climate, and are useful for dendrochronology. They determined a series intercorrelation of 0.45 and average mean sensitivity of 0.465 from a master chronology of 14 tree species. The dendroclimate group recorded a significant positive relationship (r-value = 0.589, p-value = 0.0005) between ring width of a mixed species chronology of B. longifolia and J. paniculata, and precipitation totals for Zambia’s wet season (October–April). The group that worked on dendrochemistry found that arsenic, barium, calcium, lead, zinc, manganese, and strontium bio-accumulate in B. longifolia. Workshop participants also worked on a number of trees from the wet miombo woodlands, which included the common species of Brachystegia, Julbernardia and Isoberlinia to understand their anatomical properties. The species were found to be diffuse porous.

ACKNOWLEDGEMENTS

The organizers want to thank the facilitators, PAGES, Copperbelt University, CBU-ACESM, Indiana State University, University of Cambridge, Association for Tree-ring Research, Brigham Young University, African Explosive Limited, and Cybis Elektronik & Data AB.

affiliationS

contact

Justine Ngoma: justinangoma@yahoo.com

references

Holmes RL (1983) Tree-Ring Bull 43: 69-78

Maxwell RS, Larsson LA (2021) Dendrochronologia 67: 125841

Speer JH et al. (2006) Professional Paper of the Indiana State University, Department of Geography, Geology and Anthropology 23: 3-14

The 5th Summer School on Speleothem Science (S4), São Paulo, Brazil, 6–13 August 2023

About S4

Speleothem science is integral to reconstructing past climates and environmental and archaeological change. S4 is a student-organized training school taught by leading experts, that provides an informal space to share knowledge on traditional and state-of-the-art research and methods applied in the field. Participants share and discuss their own research with peers and get valuable feedback from experts. The event is particularly suited for MSc and PhD candidates whose research interests are closely related to speleothem science. After four successful summer schools held in European cities (Heidelberg 2013, Oxford 2015, Burgos 2017, and Cluj-Napoca 2019) the current organizing committee made it a priority to significantly increase S4’s diversity and accessibility with a major geographical relocation of S4 2023 to Brazil.

The current school

The 5th S4 was hosted at the Institute of Geosciences, University of São Paulo (USP). This year’s summer school program was full-size again, after a long COVID break of four years, during which we organized two small S4 events – S4 Online, and MiniS4 2022, Innsbruck, Austria. The organizing committee that was formed in 2019 after the summer school in Cluj-Napoca was strengthened by new members recruited during MiniS4.

A total of 59 students, including bachelor, masters, PhD and postdoctoral researchers, from 25 countries participated: 28% of the students came from South America, 24% from North America, 23% from Europe, 18% from Asia and 7% from Africa (Fig. 1). We are happy to share that the proportion of male and female participants was exactly 50%. The accessibility was greatly enhanced by support from several funding sources, including PAGES, that helped to lower the registration costs by 50%. Additionally, 19 partial or full travel grants and 17 registration fee waivers were awarded.

Figure 1: Overview of participation in the 5th S4. Image credit: Julio Cauhy Rodrigues.

The scientific program consisted of two main parts. The first three days were dedicated to lectures and workshops at the Institute of Geosciences at USP. We started with an overview of speleothem-paleoclimate research in South America, followed by lectures on speleothem growth, radiogenic dating methods, and the use of proxies as stable isotopes and trace elements. The paleothermometry lecture discussed the use of fluid inclusions and clumped isotopes, as well as useful tips for their application. Substantial time was dedicated to speleothem petrography, with a lecture and workshop where participants learned novel and time-efficient approaches for using petrography as a paleoclimate proxy. Among the less traditional methods, attendees enjoyed the speleothem paleomagnetism lecture, followed by a tour of the paleomag laboratory. Further, participants learned about speleothems and climate models, attended an interesting workshop on time-series analysis, and reviewed the multi-proxy approach in paleoclimatology. The summer school concluded with a lecture on the SISAL database, its history and use, and an invitation to join this great effort of the speleothem community! We sincerely thank all the experts who participated and gave fascinating lectures for the new generation of speleothem scientists.

Fieldtrip

Following the three-day lecture portion, we traveled to the Alto Ribeira Touristic State Park for a field trip to the beautiful and large caves of the Mata Atlântica rainforest. The participants visited six Precambrian limestones caves, and discussed their genesis, the formation of speleothems, and deposition of cave sediments and emphasized ethical and environmentally friendly sampling methods. In the evenings, we organized lectures on cave monitoring with practical examples, a lecture on diversity, equity and inclusion in speleothem science, a lecture on the geomorphology and genesis of caves, and a discussion on ethics in field research, focusing on the topic of scientific neocolonialism.

To provide participants with resources for later reference, we recorded all lectures; the recordings are available on our YouTube channel (youtube.com/@speleothemsciencesummersch5335). Traditionally, after each S4 summer school, a new organizing committee is formed by the participants. News and information about the upcoming 6th Summer School on Speleothem Science can be found on the event webpage: speleothemschool.com.

ACKNOWLEDGEMENTS

We acknowledge financial support from the Centro Nacional de Pesquisa e Conservação de Cavernas (CECAV), PAGES (agreement REF-39-728), the São Paulo Research Foundation (FAPESP), and the Institute of Geosciences USP.

affiliationS

1Department of Air Quality, Slovak Hydrometeorological Institute, Bratislava, Slovakia

2Institute for Geosciences, Johannes-Gutenberg-Universität, Mainz, Germany

3Department of Earth System Science, University of California, Irvine, USA

4Institute of Geosciences, University of São Paulo, Brazil

5Department of Earth Science, University of Bergen, Norway

6Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, USA

7School of Geography, University of Melbourne, Australia

contact

Petronela Ševčíková: nela.filipcikova@gmail.com

Dr. D. Ricardo Ortega-Rodriguez, from Brazil, traveled to the Instituto de Investigación de Zonas Desérticas of Universidad Autónoma de San Luis Potosí, Mexico, as a PAGES-IAI Mobility Research Fellow from 6 August to 6 November 2023. Under the coordination of Dr. Laura Yáñez, he evaluated the potential of interannual variations of the anatomical-elements proportion of Cedrella fissilis to study climate variability.

A decrease in precipitation, and an increase in temperature and evapotranspiration, has been observed in the southern region of the Amazon basin since 1970 CE, as well as an increase in the length and severity of the dry season since 1990 CE (Fig. 1a). These extreme conditions suggest an alarming future, with possible forest dieback (Lovejoy and Nobre 2018). In this context, timeseries studies of growth-ring parameters (dendrochronology) are an essential tool to understanding the climate variability and periodicity of extreme events on an annual scale, and possible adaptive processes of species.

Figure 1: (A) Bar plots show anomalies (standardized values) of total annual precipitation (P), total annual difference between precipitation and potential evapotranspiration (P-PET), and mean annual temperature (T). (B) Mean raw series with ± standard error, mean index residual chronologies (black lines) and number of trees and radii (bottom gray shading) of the proportion of vessel (VA%), parenchyma (PA%) and fiber area (FA%) in tree rings, and boxplot of normalized parameters in dry (red) and wet (blue) years (significant difference between means, SD, p < 0.05).

Background of the dendroclimatic analysis of Cedrela fissilis

The C. fissilis population analyzed during this fellowship was collected in Rondonia, Brazil, in the southern region of the Amazon basin. Previous studies of this population show that the increase in the frequency of droughts after 1990 CE led to a long-term trend of decreased growth, and an increase in wood density (Ortega Rodriguez et al. 2023a). Furthermore, during dry years, C. fissilis showed narrower, less-dense rings, lower concentrations of S and Ca, and higher δ18O (the opposite was found in wet years; Ortega Rodriguez et al. 2023b). The observed long- and short-term adaptation by this species requires a better understanding of the trade-off processes between structural, transport and storage tissues. This can be achieved by using quantitative wood-anatomy methods.

Quantitative wood anatomy as a climate proxy

We obtained gray-tone images of 11 C. fissilis individuals (14 radial samples) through X-ray densitometry (Quintilhan et al. 2021), and classified and measured the proportion (%) of vessel (VA, a transport tissue), fiber (a structural tissue, FA) and parenchyma (a storage tissue, PA) areas in ImageJ software. Then, we obtained a timeseries (1970–2018 CE) of these anatomical parameters. We also evaluated the cross-dating among series using the COFECHA program and removed the age-size-related and non-climatic trends using the ARSTAN program.

Raw series trend showed a significant (p < 0.05) increase in VA% (r = 0.38) and FA% (r = 0.54) similar to the increase observed in RD; while PA (r = -0.70) showed a decrease, mainly since 2000 CE, similar to the trend observed in RW (Fig. 1b) (Ortega Rodriguez et al. 2023a). Furthermore, during dry years there was a higher and lower VA% and FA% in tree rings, respectively, without significant alterations in the PA%.

Contrary to expectations, there were no significant correlations between anatomical variables and precipitation, temperature, or the difference between precipitation and potential evapotranspiration (P-PET). Positive and significant correlations between the PA% and the standardized precipitation-evapotranspiration index (SPEI) values at the beginning of the rainy season (September–November) of a previous year were identified. In contrast, the FA% showed positive, and significant, correlations with the SPEI values of the end of the rainy season (March–May) of a previous year and the beginning of the rainy season (September–November) of the current year.

Final comments

The SPEI signal recorded in the PA% and fibers may suggest the importance of these variables for compensation strategies between storage tissues and structural support formed by C. fissilis in response to dry conditions. Furthermore, the opposite trend of both parameters suggests a vulnerability of the normal growth and wood density conditions of the species, in response to climate change, mainly since 2000 CE, when extreme droughts became more frequent in the region.

affiliationS

1Departamento Ciencias Florestais, Universidade de São Paulo, Piracicaba, Brazil

2Departamento de Fisica Ambietal, Universidade Federal de Mato Grosso, Brazil

3Escuela Profesional de Ingeniería Forestal, Universidad Nacional Toribio Rodríguez de Mendoza de Amazonas, Chachapoyas, Peru

4Instituto de Investigación de Zonas Desérticas, Universidad Autónoma de San Luis Potosí, Mexico

contact

D. Ricardo Ortega-Rodriguez: dai.ricardo.or@gmail.com

REFERENCES

Lovejoy TE, Nobre C (2018) Sci Adv: eaat2340

Ortega Rodriguez DR et al. (2023a) Agri For Meteorol: 333

Ortega Rodriguez DR et al. (2023b) Sci Total Environ 871: 162064

Quintilhan MT et al. (2021) Dendrochronologia: 125878

Dr. Gabriela Torre, from Argentina, visited the Universidade de São Paulo, Brazil, as a PAGES-IAI International Mobility Research Fellow (October–December 2023) to determine the age of eolian records through Optically Stimulated Luminescence (OSL). The new ages enabled determination of the chronological framework of dust deposition in southern South America, together with dust fluxes in the region. This project aims to expand understanding of the dynamics of atmospheric dust, and its influence on past climatic variabilities.

Past climatic fluctuations recorded in ice cores indicate great variability in past fluxes of atmospheric dust, emphasizing their key role in changes in the global climate system. Atmospheric dust is an important component of the climate system, affecting and responding to climate changes through a series of complex feedbacks involving nutrient cycling, albedo, radiative forcing, and cloud formation (Bullard et al. 2016). However, the role of dust in climate forcing remains poorly understood, and represents one of the largest uncertainties in climate-model simulations (Adebiyi and Kok 2020; Heavens et al. 2012). Atmospheric dust generated on the continents is a fairly unknown variable of the climate system, and loess sequences are the main records of continental dust deposition. In this sense, the loess-paleosol sequences from Argentina are the most extensive paleorecord of eolian material in the Southern Hemisphere.

The fellowship investigated the dust cycle recorded in loess-paleosols through the determination of dust accumulation rates for the past glacial–interglacial periods.The comparison between dust fluxes at the continental loess deposits with those recorded at more distal regions (south Atlantic Ocean and Antarctic Plateau) will allow us to improve our knowledge related to the dynamics of dust during the last glacial–interglacial cycle. We applied OSL techniques in two loess-paleosol sequences, located in elevated regions, to increase the spatial resolution of previous studies in dust archives (Torre et al. 2022), and to enhance the spatial resolution of paleoclimatic studies in the continental region of southern South America.

Research activities

1) Sample selection: A total of 20 samples were collected for OSL determinations from two loess-paleosol sections exposed in relatively elevated regions – Las Carreras section at ~2290 m asl and Majada de Santiago section at 1600 m asl (Fig. 1). These sections were sampled vertically every 20 cm for the uppermost 2 m by embedding PVC pipes 1.5" in diameter.

Figure 1: Map showing the location of the two loess sections studied in this work (white circles): Tucumán section and Córdoba section. The yellow circles indicate previously studied loess sections within the Pampean Plain.

2) Equivalent dose determination: All sample preparation was performed in a lab room with red/orange light conditions. Samples were open and sieved to separate fine sand from silt-size particles. Then, samples were treated with HCl and H2O2 hydrogen peroxide to eliminate carbonates and organic matter. Thereafter, quartz grains were isolated through heavy liquid separation methods. The concentrates were subjected to HF attack for 40 minutes. This procedure removes remanent feldspar grains and the outer ring of quartz grains.

3) Dose rate determination: Dose-rate calculation was performed through the determination of 238U, 232Th and K concentrations. For this purpose, samples were dried and stored in plastic containers previously sealed for more than 20 days. Dose rate was determined through high-resolution gamma-ray spectrometry with a high-purity germanium detector and corrected for moisture content (water weight/dry sample weight) of each sample. Also, dose rate was corrected for the cosmic rays dose rate, which is estimated as a function of depth, altitude and latitude of each sample.

Outcomes

Infrared stimulation tests showed a significant contamination of feldspar in the quartz concentrate samples even after several HF attacks. Due to reduced sample size, and difficulty in obtaining pure quartz aliquots, luminescence dating had to be applied in silt-sized polymineral aliquots. Toward the end the fellowship, the 4-11 µm fraction was isolated through settling in distilled water, followed by acid attacks. Equivalent doses of the silt fraction were measured in December 2023 and January 2024.

Once the measurements of dose rate and equivalent doses for each sample are ready, we will be able to calculate the age of deposition of each level (i.e. age(yr)= equivalent dose(Gy)/dose rate (Gy yr−1)). These ages will provide a chronological framework of the different geochemical and physical parameters already determined for the loess samples. Also, these ages will allow the correlation of loess sections with other paleoclimatic records of the Southern Hemisphere, improving the spatial resolution of paleoclimatic studies. Moreover, the detailed chronology will allow the determination of the mass-accumulation rates of eolian sediments. This proxy will be a valuable contribution to the modeling community working on the past glacial cycle, and will also allow us to compare dust fluxes between different dust records, which surely will improve the understanding of past dust variability.

affiliationS

1Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Argentina

2Nacional de Investigaciones Científicas y Tecnológicas (CONICET), Centro de Investigaciones en Ciencias de la Tierra (CICTERRA), Córdoba, Argentina

3Instituto de Geociencias, Universidade de São Paulo, Brazil

contact

Gabriela Torre: gabrielatorre@unc.edu.ar

REFERENCES

Adebiyi AA, Kok JF (2020) Sci Adv 6: eaaz9507

Bullard JE et al. (2016) Rev Geophys 54(2): 447-485

Heavens NG et al. (2012) J Adv Mod Earth Syst 4(4): M11002

Torre G et al. (2022) Earth-Sci Rev 232: 104143

Lic. Rodrigo Martín, from Argentina, visited the University of São Paulo, Brazil, as a PAGES-IAI International Mobility Research Fellow (23 July – 27 Aug 2023) to perform X-ray fluorescence (XRF) and stable isotope analyses on sediment cores retrieved from the western South Atlantic Ocean. These analyses will help to understand the sediment sources and transport pathways active in the region since Termination I.

The western South Atlantic circulation is dominated by the Brazil-Malvinas Confluence (~38ºS) that emerges from the encounter of the southward-flowing Brazil Current and the equatorward-flowing Malvinas Current. The upper slope off Uruguay, however, is strongly influenced by the Brazilian Coastal Current that transports the Plata Plume Water (PPW), derived from the Plata River discharge (Fig. 1a). The northward penetration of the PPW is mainly controlled by the seasonal along-shore wind stress, including the southern westerly winds (SWW), and reaches its northernmost position during the austral winter when the SWW displace to the north (Piola et al. 2008). South of 38ºS, the Patagonian margin is controlled by the Malvinas Current, and the low precipitation and strong SWW over the Patagonian Steppe make it a major dust source to the adjacent ocean (Prospero et al. 2002).

The past evolution of the sources and transport pathways of terrigenous sediments deposited at the western South Atlantic since Termination I is poorly understood. To shed light on this issue and provide important insights into the climate and oceanographic variability in this region, we analyzed three radiocarbon-dated marine sediment cores from the Uruguayan and Patagonian margins (Fig. 1a). We used major elemental composition to trace changes in the terrigenous input: Fe/Ca ratio employed as an indicator of terrigenous material of fluvial origin (Arz et al. 1998), and Ti/Al ratio used as an indicator of grain size variation and/or provenance (Govin et al. 2012). Additionally, we analyzed the benthic-foraminifera stable carbon isotopes (δ13C) to gain insight into intermediate water circulation.

Figure 1: (A) Map showing annual sea-surface temperature of the western South Atlantic. White diamonds represent the analyzed cores. White arrows represent the main oceanic currents (BC: Brazil Current; BCC: Brazilian Coastal Current; PPW: Plata Plume Water). Gray arrows represent the southern westerly winds (SWW). BMC: Brazil-Malvinas Confluence. (B) Records of benthic-foraminifera stable carbon isotopes (Cibicidoides-δ13C), ln(Fe/Ca) and ln(Ti/Al) ratios of cores GeoB22735-2 (Uruguayan margin), AU_Geo02_GC20 and AU_Geo02_GC21 (Patagonian margin).

Terrigenous input evolution from the western South Atlantic

Our geochemical data indicate that the terrigenous input to the western South Atlantic was higher during Termination I than during the Holocene (Fig. 1b). The lower sea level, together with a northward displacement of the SWW, increased terrigenous material input at the Uruguayan (GeoB22735-2) and Patagonian (AU_Geo02_GC20/21) margins. Nevertheless, the sedimentary pathways at these two regions were different.

At the Patagonian margin, low Ti/Al ratios suggest low eolian input; and high Fe/Ca ratios reflect not only lower sea level, but also enhanced input of fluvial-derived material due to melting of the Patagonian Ice Sheet (Gaiero et al. 2003). At the Uruguayan margin, conversely, high Fe/Ca ratios reflect a major influence of the PPW due to a northern position of the SWW (Piola et al. 2008), whereas the low Ti/Al ratios indicate limited input from the Bermejo River (a major Andean tributary) to the Plata River (Depetris et al. 2003).

During the Holocene, the decrease in Fe/Ca ratios indicates a reduction in the terrigenous input of fluvial origin at both sectors due to sea-level rise and the southward displacement of the SWW. Higher Ti/Al ratios at the Patagonian margin suggest a stronger Patagonian dust plume, whereas at the Uruguayan margin it indicates a major relative contribution of sediments derived from the Bermejo River that carry a signature rich in Ti (Depetris et al. 2003). Preliminary analyses of the Holocene δ13C indicate that the higher values recorded at the Uruguayan margin since the mid-Holocene indicate a smaller influence of southern-source waters at intermediate depths (García Chapori et al. 2022). Further ongoing analyses will help to evaluate these hypotheses in this key region of the worlds oceans.

ACKNOWLEDGEMENTS

We thank Prof. Dr. Kasten (AWI, Germany) and Dr. Tassone (IGEBA, Argentina) for providing the sediment cores analyzed here. The study was supported by the PAGES-IAI International Mobility Research Fellowship and Project UBACyT-20020190100204BA.

affiliationS

1Institute of Andean Studies, University of Buenos Aires-CONICET, Argentina

2School of Arts, Sciences and Humanities, University of São Paulo, Brazil

contact

Rodrigo S. Martín: rodrigosmartin88@gmail.com

references

Arz HW et al. (1998) Quat Res 50(2) : 157-166

Depetris PJ et al. (2003) Hydr Proc 17(7): 1267-1277

Gaiero DM et al. (2003) Geoch Cosm Acta 67(19): 3603-3623

García Chapori N et al. (2022) J South Amer Earth Sci 117: 103896

Govin A et al. (2012) Geoch, Geoph, Geosy 13(1): Q01013

Piola AR et al. (2008) Cont Shelf Res 28(13): 1639-1648

Prospero JM et al. (2002) Rev Geoph 40(1): 1002

Dr. Jorge A. Giraldo, from Colombia, visited the Laboratorio de Dendrocronología de Zonas Áridas, Ladeza-CIGEOBIO (CONICET-UNSJ) in San Juan, Argentina, as a PAGES-IAI International Mobility Research Fellow (1 June–31 July 2023) to explore the dendrochronological potential of tree species from xeric ecosystems in South America to record vapor pressure deficit (VPD). Within this project, Jorge and his collaborators aim to increase knowledge of the long-term variability of VPD and its effects on tree growth over time.

Motivation

The sensitivity of forests to climate change in South America has been of interest to the scientific community over the past decades (Morales et al. 2020; Villalba et al. 2011). However, the scarcity of climatic records from field stations in this region limits our ability to detect the effect of climate change (Garreaud et al. 2009). Dendrochronology can provide valuable climate proxy records over long periods derived from annual tree rings. Therefore, extensive geographic sampling of old trees across an ecosystem can improve current climate databases and refine our inference ability about the role of climate on South American forests (Morales et al. 2020). Under global warming, air dryness (i.e. vapor pressure deficit: VPD) has markedly increased around the globe (Grossiord et al. 2020). VPD is a multidimensional variable combining temperature and relative humidity, enabling measurement of the atmospheric water demand from plants (Yuan et al. 2019). While much attention has been directed towards plant responses to temperature and precipitation independently (i.e. reconstructions of these variables), few studies have isolated the response of plant functioning toward reconstructions of VPD variability using tree-ring width.

Thanks to the support from this mobility research fellowship, it was possible to strengthen a collaborative network between tree-ring research groups from Colombia and Argentina, enabling the investigation of the dendrochronological potential of Araucaria araucana, a tree species from the northwest of Patagonia, to reconstruct VPD variation using tree-ring width.

Figure 1: (A) Location of the tree-ring sampling; and (B) combined ring width index (RWI) chronology for three sites. SSS: subsample signal strength. (C) Graphical relationship between RWI and mean January–February VPD over time and its correlation divided into calibration and verification period. (D) Scatter plot of the relationship between RWI and the mean January–February VPD.

Analysis

Applying standard dendrochronology techniques (i.e statistical and graphical cross dating), we analyzed available samples from three populations of A. araucana growing in xeric sites in Argentina (i.e. 131 trees and 244 cores; Fig. 1a), collected by Dr. M. Hadad and colleagues. The similarity between sites (i.e. climate and topography) and the crossdating allowed us to combine them into a single representative chronology for the area (Fig. 1b). Although the oldest tree dates back to 1190 CE, we built a chronology represented by more than five tree series which span from 1377–2019 CE. The measured subsample signal strength (SSS), which is a measure of the variance in common between a subset of samples and master chronology, was higher than 0.85, suggesting the suitability of the dendrochronological chronology for climate reconstructions (1588–2019 CE; Fig. 1b). We compared tree-ring chronology with VPD series estimated from the Climate Research Unit products (CRU). We found significant (p < 0.05) correlations between tree-ring chronology and monthly VPD from December (r = -0.18), January (r = -0.38), February (r = -0.35), March (r = -0.30), and the mean January–February VPD (r = -0.46) of the previous growing season. We compared the tree-ring width index to the mean January–February VPD record using the split calibration/verification method to test the reconstruction potential of this species. The chronology accounted for 55% of the variance in the calibration period (1982–2016 CE; r = -0.76, p < 0.05; Fig. 1c), while the full calibration period (1969–2016 CE) explained 44% of the variance (Fig. 1d). In addition, the positive values of reduction of error (RE: 0.64), and the coefficient of efficiency (CE: 0.36) indicate a stability of the relationship, which suggests the suitability of the chronology to be used in reconstructions of VPD back in time.

In conclusion, the results of this mobility research fellowship demonstrate a strong potential of A. araucana growing in xeric sites of South America to reconstruct VPD during its growing season. Currently, we are preparing a manuscript to show a reconstruction of the VPD following standard methods. This can improve our understanding of regional/continental VPD variation in Northern Patagonia under the ongoing climate situation.

ACKNOWLEDGEMENTS

The sampled collection was supported by the Agencia Nacional de Promocion Cientifica y Tecnologica of Argentina (PICT-2018-1056 to MAH). We also thank Ladeza-CIGEOBIO and Tecnológico de Antioquia Institución Universitaria.

affiliationS

1Facultad de Ingeniería – Tecnológico de Antioquia Institución Universitaria, Medellín, Colombia

2Laboratorio de Dendrocronología de Zonas Áridas. CIGEOBIO (CONICET-UNSJ), Gabinete de Geología Ambiental (INGEO-UNSJ), San Juan, Argentina

3Instituto de Ciencias de la Tierra ICT, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile

4Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Mendoza, Argentina

contact

Jorge A. Giraldo: jorge.giraldo76@tdea.edu.co

references

Garreaud RD et al. (2009) Palaeogeogr Palaeoclimatol Palaeoecol 281: 180-195

Grossiord C et al. (2020) New Phytol 226: 1550-1566

Morales MS et al. (2020) Proc Natl Acad Sci USA 117: 16816-16823

Villalba R et al. (2011) In: Hughes MK et al. (Eds) Dendroclimatology. Springer: 175-227

Yuan W et al. (2019) Sci Adv 5: 1-13

Sclerosponges dwell in an underexplored region of reefs. Their massive basal skeleton offers an opportunity as a unique, but underappreciated, paleoenvironmental archive. Here, we give an overview of the paleoclimate potential of sclerosponges and highlight current and future research areas.

Sclerosponges: Dwellers of caves, overhangs and mesophotic zones

Tropical shallow-water coral reefs are colorful, bright environments in which light drives the growth of a plethora of corals and other organisms. Off the well-lit parts of the reef, however, the light gets gradually dimmer and different organisms appear. Caves, overhangs, and the twilight environment of the mesophotic zone represent the refuge for a group that, in evolutionary terms, are the most ancient animals that form a massive calcareous skeleton (Fig. 1a). These organisms are coralline sponges or sclerosponges which were thought to be extinct until their rediscovery in cryptic habitats in the early 20th century.

Figure 1: (A) A diver (G. Wörheide) samples sclerosponges in the Coral Sea, Australia. Photo credit: C. Vogler. (B) A. willeyana (yellow) off Saipan, Northern Mariana Islands. Photo credit: A. Grottoli.

While their ancient relatives contributed significantly to reef-building during the Paleozoic and Mesozoic, in today’s oceans, sclerosponges are important contributors to nutrient cycling, and their massive skeletons cement and stabilize reef structures. The depth distribution of approximately one dozen sclerosponge species spans from surface waters to over 600 m deep, and they are found in the tropical and subtropical regions of the Atlantic, Pacific and Indian oceans (Vacelet et al. 2010). Although leading a life in shaded environments, sclerosponges are important for the movement of elements between living things and the ocean water.

Sclerosponges as paleoclimate archives

Research on sclerosponges dates back to the early 20th century when Joseph Jason Lister (1857–1927) and Randolph Kirkpatrick (1863–1950) investigated "pharetronid sponges" and rediscovered "living fossils" (e.g. Astrosclera willeyana Lister, 1900 from the Pacific; Fig. 1b). It was only in 1976 that Veizer and Wendt published the first geochemical analysis of the basal skeleton of A. willeyana (Fig. 1b), Ceratoporella nicholsoni, and Petrobiona massiliana. Together with Acanthochaetetes wellsi (Fig. 2), they are currently still the most investigated sclerosponge species.

Since the first geochemical investigations in the 70s, sclerosponges have been successfully used for paleo reconstructions. Their first application as an environmental archive was presented in 1986 by Druffel and Benavides, who used sclerosponges from the Caribbean to fingerprint oceanic anthropogenic CO2 increases through δ13C analysis. The observed decrease of 13C, the 13C-Suess effect, results from fossil fuel burning. This effect was later also found in Pacific sclerosponges (Böhm et al. 1996). Radiocarbon analyses revealed the bomb curve; a peak in Δ14C due to above-ground thermonuclear testing starting in 1955 (Benavides & Druffel 1986). Instrumental advances in geochemistry allowed an increase in the spatial analytical resolution from mm- to µm-scale, which is equivalent to bimonthly resolution in sclerosponge skeletons (Swart et al. 2002). Besides their application to trace atmospheric chemistry, they can be used to reconstruct seawater temperature and δ18O values by analyzing their trace metal and stable isotope composition. While Wörheide (1998) reconstructed relative temperature changes using δ18O values, Rosenheim and coworkers (2004) provided the first absolute temperatures through an empirically calibrated Sr/Ca-temperature relationship, with the longest record from a single sponge being ≈ 600 years (Waite et al. 2020).

Figure 2: A section of A. wellsi from Saipan, Northern Mariana Islands. The sample shows an alizarin red stain from a growth-rate experiment. Photo credit: A. Grottoli.

To achieve any paleoclimate record, a chronology of the skeleton is necessary, and the sclerosponge needs to be dated. Unlike corals or bivalves, sclerosponges lack the presence of an internal annual banding that would allow dating through sclerochronological means, such as with tree-ring chronologies. Dating and growth-rate estimates of sponges have so far been conducted by in situ staining with fluorescent dyes (Fig. 2), δ13C-Suess effect values, the identification of the leaded fuel combustion-Pb peak in 1971, and Δ14C or U/Th radiometric dating (e.g. Benavides & Druffel 1986; Swart et al. 2002; Wörheide 1998). These methods are currently the only way to determine their age. Compared to tropical corals, coralline sponges are slow-growing at less than 2 mm/year (e.g. Grottoli et al. 2010). Still, they have extraordinary longevity, reaching up to one thousand years (Vacelet et al. 2010). Hence, individuals of C. nicholsoni can become larger than 1 m in diameter, emphasizing their potential as a valuable high-resolution environmental archive for extended periods (Lang et al. 1975).

More recent work focused on proxy verification and reconstructing the paleoceanographic history of the Pacific using element ratios, δ13C and δ18O values (e.g. Asami et al. 2021; Grottoli et al. 2010, 2020). These studies found little impact of biological activity, the so-called vital effect, on skeletal δ13C and δ18O values, or the partitioning of Sr, and they also reported P/Ca ratios as a promising proxy of seawater P concentrations. An improved Sr/Ca temperature calibration for C. nicholsoni allowed for water-temperature reconstruction with a precision of <0.5°C (Waite et al. 2018), enabling Waite et al. (2020) to determine how the combination of volcanic activity and anthropogenic forcings drove Atlantic climate variability during the past 600 years.

The anthropogenic change in atmospheric CO2 has lowered the pH of the ocean. Boron isotopes (skeletal δ11B), a common proxy for seawater-pH reconstruction in corals, showed that C. nicholsoni grows close to equilibrium with the seawater borate isotopic composition and links δ11B variability on the sub-mm scale to seasonal pH changes (e.g. Sadekov et al. 2019). These results emphasize the potential of sclerosponges as a recorder of ocean acidification. Sclerosponges were also applied to trace other anthropogenic pollutions. For example, Pb/Ca ratios are used to trace environmental Pb contamination (e.g. Asami et al. 2021).

In conjunction with their longevity and resistant skeleton, these recent findings concerning the geochemistry of sclerosponges pave the way for establishing them as a valuable archive of climatic, oceanographic and pollution conditions at depths beyond that recorded by scleractinian corals in warm-water reefs.

The future of paleo-sclerosponge science

The importance of the underexplored mesophotic zone makes research on local environmental archives, such as those provided by sclerosponges, urgently required. New sampling and habitat mapping opportunities using affordable underwater drones, or hybrid, remotely-operated vehicles, can increase the accessibility of coralline sponges. Technological advances in the field of omics (genomics, proteomics, metabolomics, etc.), X-ray microtomography and geochemical analytical advances like Laser Ablation Inductively Coupled Plasma Time of Flight Mass Spectrometer (LA-ICP-ToF-MS), or Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), allow for a wide range of analyses of slow-growing carbonate structures.

Recent years have seen a huge leap forward in understanding the biomineralization of scleractinian corals to build their CaCO3 skeletons. In sclerosponges, however, very little is known. Arguably, a thorough knowledge of biomineralization is required, to not only form a mechanistic understanding of the impacts of future climate change, but also as a prerequisite for the application of geochemical proxies. Since sclerosponges are among the most ancient multicellular CaCO3 biomineralizers, such insights could also provide unique information about how animal skeletal formation evolved.

So far, diagenesis has only been studied in fossil sponges, but no studies have investigated the fate of the skeletal elemental and isotopic composition and structure after formation. In addition to long-term changes, several other skeletal processes require further studies, such as the extracellular backfill of the skeleton observed in A. willeyana (Wörheide 1998) and the chemical heterogeneity in A. willeyana and C. nicholsoni that could not be linked to environmental variations (e.g. Asami 2021).

Despite these challenges and the limited number of paleo studies, sclerosponges have enormous potential as long archives of past climates in the subsurface of the upper ocean. However, to maximize the potential of these environmental archives, the following issues are pressing areas of sclerosponge investigation: i) dating at sub-seasonal resolution; ii) lab culturing and in situ experiments on the effect of environmental changes on skeletal formation and composition; iii) investigation into possible early diagenesis; and iv) the influence of biomineralization on the geochemistry of the calcareous skeleton.

One exciting aspect of sclerosponges, which was only briefly explored, is the potential to reveal changes in the thermocline structure of the upper ocean. As the ocean warms, the depth at which the thermocline occurs should deepen. Given carefully chosen sampling locations and appropriate geochemical methods, sclerosponges can offer insight into how thermocline depth and heat penetration in the oceans have responded to rising global temperatures.

Addressing these knowledge gaps would be most effective through cross-disciplinary collaboration between paleoclimatologists, (bio)geochemists, microbiologists, geneticists, and physical oceanographers. We hope this article stimulates these collaborations to maximize the study of sclerosponges as a potentially unique climate archive.

affiliations

1Department of Earth Sciences, University of Geneva, Switzerland

2School of Ocean and Earth Science, University of Southampton, UK

3National Oceanography Centre Southampton, UK

4School of Earth Sciences, The Ohio State University, USA

5Division of Marine Geology and Geophysics, Rosenstiel School of Marine, Atmospheric, and Earth Sciences, University of Miami, USA

6Kravis Department of Integrated Sciences, Claremont McKenna College, USA

7Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Germany

contact

Sebastian Flöter: sebastian.floeter@unige.ch

references

Asami R et al. (2021) Prog in Earth Planet Sci 8: 38

Böhm F et al. (1996) Earth Planet Sci Lett 139: 291-303

Benavides LM, Druffel ERM (1986) Coral Reefs 4(4): 221-224

Druffel ERM, Benavides LM (1986) Nature 321: 58-61

Grottoli AG et al. (2010) J Geophys Res 115: 1-14

Grottoli AG et al. (2020) Geochem Geophys Geosyst 21: 1-14

Lang JC et al. (1975) J Mar Res 33: 223-231

Rosenheim BE et al. (2004) Geology 32: 145-148

Sadekov A et al. (2019) J Anal Atom Spectrom 34: 550-560

Swart PK et al. (2002) Paleoceanography 17: 1-12

Vacelet J et al. (2010) Treatise Invertebr Paleontol 4: 1-16

Veizer J, Wendt J (1976) J Archäol Sci 23: 558-573

Waite AJ et al. (2018) Chem Geol 488: 56-61

Waite AJ et al. (2020) Geophys Res Lett 47: 1-11

Wörheide G (1998) Facies 38: 1-88

A new reconstruction combines an unprecedented amount of climate observations with 12,000 years of climate model simulations in a data assimilation approach.

Reconstructions using data assimilation

Understanding interannual-to-decadal climate variability and extremes requires long datasets. Climate reconstructions have provided this information for decades. However, to understand the underlying mechanisms and relate climatic changes to atmospheric circulation variability, more comprehensive datasets than hitherto available are essential. One way of achieving this is to assimilate as much high-resolution climate information as possible into climate model simulations that provide physically consistent climate states.

Data assimilation (DA) approaches combining real-world atmospheric information with the physics of a climate model have been extremely successful in atmospheric sciences. In paleoclimatology, they are increasingly used to reconstruct the climate of the last millennium (Franke et al. 2017; Goosse and Paul 2013; Tardif et al. 2019). In DA, a first guess (mostly a modeled climate state) is updated by observations to provide the best estimate of climate consistent with both the model and the observations, as well as consistent with the errors in both. In paleoclimatology, offline DA is often used. In contrast to the online version, in which the updated model state at every assimilation time step is used to initialize a new forecast, offline DA only performs the update. This is possible because for atmospheric models, and over the typically seasonal-to-annual timescale of the assimilation, atmospheric initial conditions hardly matter. Offline DA is simple and versatile and can be used with pre-existing model runs. The latest offline DA product is the Modern Era Reanalysis (ModE-RA) suite of products (Valler et al. 2024). ModE-RA provides monthly output using a seasonal assimilation window, such that monthly mean instrumental or documentary data can be assimilated simultaneously with seasonal proxies (e.g. tree rings, plant or ice phenology).

Comprehensive compilation of observations

Crucially, any climate reconstruction relies on sufficient input data. This is the key strength of ModE-RA, which makes use of many times more observations than previous reconstructions. In addition to a large set of tree ring and other natural proxy records that include, among others, the PAGES 2k Network database (PAGES2k Consortium 2017), ModE-RA uses an extended set of documentary proxies (Burgdorf et al. 2023) and a new set of early instrumental measurements (Lundstad et al. 2023). The latter is a systematic compilation of most of the available station data (air pressure, surface-air temperature, and precipitation), supplemented by ca. 1500 newly rescued series. Furthermore, air pressure measurements from ships were assimilated (Freeman et al. 2017). An overview of the data sources is given in figure 1. In total, ModE-RA assimilates hundreds (17th century), thousands (18th century) or tens of thousands (19th century) of values per year. Already in the mid-18th century, the number of values from instrumental and documentary data exceeds that of natural proxies. Only series starting before 1890 CE are included. Therefore, the number of series drops off steeply towards the 21st century as series that end are not replaced. This was done to keep the network stable and comparable over a long time period, and to limit the computation time.

Figure 1: Number of values assimilated per year into ModE-RA, separated into data types.

The observations are assimilated into an ensemble of 20 atmospheric model simulations (Hand et al. 2023) performed with ECHAM6 and providing data at ca. 2°x2° resolution. These simulations, termed ModE-Sim, were externally forced by greenhouse gas concentrations, solar irradiance and volcanic aerosols, according to the PMIP4 protocol (Jungclaus et al. 2017). Sea-surface temperature (SST) boundary conditions are based on the annual PAGES 2k Network SST reconstructions (Neukom et al. 2019) and augmented by adding intra-annual variability based on ocean-variability modes. After 1780 CE we also assimilated SST and marine-air temperature observations into the ensemble of SST reconstructions using offline DA (Samakinwa et al. 2021).

The offline DA technique is similar to the precursor product EKF400v2, with some improvements (Valler et al. 2024). We use an Ensemble Kalman Filter approach, with a hybrid background error covariance matrix that blends the ensemble covariance matrix with a climatological covariance matrix. Each of the 20 members of ModE-Sim is updated, hence ModE-RA is an ensemble of 20 members. DA is performed on anomalies from a 71-year moving average, which is added back at the end. This means that on multidecadal to centennial timescales the assimilation does not add much information, and ModE-RA becomes similar to ModE-Sim.

Instrumental data are not homogenized before assimilation, rather, a breakpoint detection is used to segment the data. The assimilation is then performed in three cycles, progressing from long (>50 year) to short (<5 year) series. Intermediate analyses are stored, and the next shorter series are then debiased, relative to this product. This allows for the use of even shorter series, of which there are many.

Example analyses

Table 1: ModE-RA products (n = number of members, obs. = assimilation of observations, *ModE-Sim has additional members not used for ModE-RA).

In addition to the product ModE-RA in which the model simulations for a given year are updated using observations from that year, we also provide ModE-RAclim. In this version, the first guess consists of a random sample of n = 100 from all years and members. The covariance matrix is constructed from the same sample. This means that ModE-RAclim is an ensemble of 100 members and does not see the time-varying boundary conditions of the model. Analyzing all three products together (ModE-RA, ModE-RAclim and ModE-Sim; see Table 1) allows for disentangling where the information in ModE-RA comes from. This is shown in figure 2a–c, which plots ensemble mean temperature anomalies over Europe in December 1783. The model simulations (Fig. 2c) indicate a 2°C north-south gradient. ModE-RAclim and ModE-RA both show a cooling over Eastern Europe, whereas the Iberian Peninsula is warmer than normal. In this case, most of the information comes from the large number of observations. However, over North Africa, ModE-RA, which sees the boundary conditions, is cooler and more similar to ModE-Sim than ModE-RAclim.

Figure 1: Temperature anomalies (from 1781–1810 CE) in December 1783 CE in (A) ModE-RA; (B) ModE-RAclim; and (C) ModE-Sim. (D) 500 hPa geopotential height in December 1783 CE in ModE-RA. (E) Anomalies of precipitation and 850 hPa wind in June–August 1809 CE (relative to 1781–1810 CE) in Central and Northern Africa and (F) 500 hPa geopotential height over the USA in April–September 1443–1445 CE (relative to 1421–1450 CE). Symbols indicate the assimilated observations. Only the ensemble means are displayed (except in the middle row, where light blue lines show members).

Only the ensemble means are shown in the top row of figure 2, but the 20 (ModE-RA, ModE-Sim) or 100 (ModE-RAclim) members can also be analyzed individually. This is highlighted for 500 hPa geopotential height for the same month in figure 2d. Individual ensemble members provide a more realistic variability over time, whereas the variability of the ensemble mean decreases backward in time, when fewer observations constrain the reanalysis.

Precipitation anomalies in Africa in 1809 CE, after the "unknown" volcanic eruption (Timmreck et al. 2021), indicate a decrease in precipitation due to a decrease of the African monsoon, which can be diagnosed in the 850 hPa wind anomaly field (Fig. 2e). Finally, a 3-year drought period in the 15th century in the USA is studied (Fig. 2f). The 500 hPa geopotential height anomalies indicate a strengthened Great Plains Ridge.

An important feature of the ModE-RA products is the Observation Feedback Archive, which provides detailed information about each observation that has been considered for assimilation. For instance, it includes station coordinates, 71-year climatology and anomaly, background and analysis departures for all members, forward models, and any relevant quality information.

The new dataset provides the best estimate of monthly climate of the past 600 years, given everything we know; the forcings, the laws of physics and arguably the most comprehensive set of climate information ever used in a reconstruction approach. It builds on efforts from several PAGES groups such as PAGES 2k Network, CRIAS and VICS and will benefit many future analyses. However, it should be noted that on multidecadal-to-centennial scales, the reconstruction mostly follows the model simulations.

ACKNOWLEDGEMENTS

Funded by the European Union H2020/ERC grant 787574, MSCA grant 894064 and Swiss National Science Foundation grant 188701. The simulations were performed at the Swiss Supercomputer Centre (CSCS).

affiliations

*Stefan Brönnimann, Yuri Brugnara, Angela-Maria Burgdorf, Jörg Franke, Ralf Hand, Laura Lipfert, Elin Lundstad, Eric Samakinwa, Veronika Valler

Institute of Geography and Oeschger Centre for Climate Change Research, University of Bern, Switzerland

contact

Stefan Brönnimann: stefan.broennimann@unibe.ch

references

Burgdorf A-M et al. (2023) Sci Data 10: 402

Franke J et al. (2017) Sci Data 4: 170076

Freeman E et al. (2017) Int J Climatol 37: 2211-2232

Goosse H, Paul A (2013) PAGES News 21: 71-79

Hand R et al. (2023) Geosci Model Dev 16: 4853-4866

Jungclaus JH et al. (2017) Geosci Model Dev 10: 4005-4033

Lundstad E et al. (2023) Sci Data 10: 44

Neukom R et al. (2019) Nature 571: 550-554

PAGES2k Consortium (2017) Sci Data 4: 170088

Samakinwa E et al. (2021) Sci Data 8: 261

Tardif R et al. (2019) Clim Past 15: 1251-1273

Timmreck C et al. (2021) Clim Past 17: 1455-1482

Valler V et al. (2024) Sci Data 11: 36