A. Bahr, A. Jaeschke, A. Hou, K. Meier, C. Chiessi, A. L. Spadano Albuquerque, J. Rethemeyer, O. Friedrich
The reconstruction of accurate sea‐surface temperatures (SST) is of utmost importance due to the ocean's central role in the global climate system. Yet, a number of environmental processes might bias reliable SST estimations. Here, we investigate the fidelity of SST reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5, utilizing a core collected off eastern Brazil at ∼20°S. This interval was selected as previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a peculiar pooling of warm surface waters in the WTSA during MIS 6 despite glacial boundary conditions. To ground‐truth the Mg/Ca‐based SST data we generated SST reconstructions on the same core material using the alkenone and TEX86 paleothermometers. Comparison with alkenone‐based temperature estimates corroborate the previous Mg/Ca‐based SST reconstructions, supporting the suggestion of a warm‐water anomaly during MIS 6. In contrast, TEX86‐derived temperatures, albeit representing annual mean SST in recent core top samples, are up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. We interpret the periods of anomalously cold TEX86‐temperatures as a result of a vertical migration of the TEX86 producers (heterotrophic marine Thaumarchaeota) toward greater water depths, following food availability during phases of enhanced fluvial suspension input. Likewise, the data suggest that alkenone‐based SST are, albeit to a minor degree when compared to TEX86, affected by river run‐off and/or a seasonal bias in the growth season of haptophyte algae.
{"title":"A Comparison Study of Mg/Ca‐, Alkenone‐, and TEX86‐Derived Temperatures for the Brazilian Margin","authors":"A. Bahr, A. Jaeschke, A. Hou, K. Meier, C. Chiessi, A. L. Spadano Albuquerque, J. Rethemeyer, O. Friedrich","doi":"10.1029/2023PA004618","DOIUrl":"https://doi.org/10.1029/2023PA004618","url":null,"abstract":"The reconstruction of accurate sea‐surface temperatures (SST) is of utmost importance due to the ocean's central role in the global climate system. Yet, a number of environmental processes might bias reliable SST estimations. Here, we investigate the fidelity of SST reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5, utilizing a core collected off eastern Brazil at ∼20°S. This interval was selected as previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a peculiar pooling of warm surface waters in the WTSA during MIS 6 despite glacial boundary conditions. To ground‐truth the Mg/Ca‐based SST data we generated SST reconstructions on the same core material using the alkenone and TEX86 paleothermometers. Comparison with alkenone‐based temperature estimates corroborate the previous Mg/Ca‐based SST reconstructions, supporting the suggestion of a warm‐water anomaly during MIS 6. In contrast, TEX86‐derived temperatures, albeit representing annual mean SST in recent core top samples, are up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. We interpret the periods of anomalously cold TEX86‐temperatures as a result of a vertical migration of the TEX86 producers (heterotrophic marine Thaumarchaeota) toward greater water depths, following food availability during phases of enhanced fluvial suspension input. Likewise, the data suggest that alkenone‐based SST are, albeit to a minor degree when compared to TEX86, affected by river run‐off and/or a seasonal bias in the growth season of haptophyte algae.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49402112","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
E. Beisel, N. Frank, L. Robinson, Marleen Lausecker, R. Friedrich, S. Therre, A. Schröder‐Ritzrau, M. Butzin
The radiocarbon analysis of uranium‐thorium‐dated cold‐water corals (CWCs) provides an excellent opportunity for qualitative reconstruction of past ocean circulation and water mass aging. While mid‐depth water mass aging has been studied in the Atlantic Ocean, the evolution of the thermocline is still largely unknown. Here we present a combined 14C and 230Th/U age record obtained from thermocline dwelling CWCs at various sites in the eastern Atlantic Ocean, with intermittently centennial resolution over the last 32 ka. Shallow dwelling CWCs off Angola, located in the South Atlantic, infer a link between the mid‐depth equatorial Atlantic and Southern Ocean. They confirm a 14C drawdown during the Last Glacial Maximum (LGM) and advocate for a consistent Southern Hemisphere radiocarbon aging of upper thermocline waters, as well as strong depth gradients and high variability. Direct comparison with 14C simulations carried out with an Ocean General Circulation Model yield good agreement for Angola. In contrast, the North Atlantic thermocline shows well‐ventilated water with strong variations near the position of today's Azores Front (AF), neither of which are captured by the model. During the Bølling‐Allerød, we confirm the important role of the AF in separating North and South Atlantic thermocline waters and provide further evidence of a 500 year long deep convection interruption within the Younger Dryas (YD). We conclude that the North and South Atlantic thermocline waters were separately acting carbon reservoirs during the LGM and subsequent deglaciation until the modern circulation was established during the YD.
{"title":"Climate Induced Thermocline Aging and Ventilation in the Eastern Atlantic Over the Last 32,000 Years","authors":"E. Beisel, N. Frank, L. Robinson, Marleen Lausecker, R. Friedrich, S. Therre, A. Schröder‐Ritzrau, M. Butzin","doi":"10.1029/2023PA004662","DOIUrl":"https://doi.org/10.1029/2023PA004662","url":null,"abstract":"The radiocarbon analysis of uranium‐thorium‐dated cold‐water corals (CWCs) provides an excellent opportunity for qualitative reconstruction of past ocean circulation and water mass aging. While mid‐depth water mass aging has been studied in the Atlantic Ocean, the evolution of the thermocline is still largely unknown. Here we present a combined 14C and 230Th/U age record obtained from thermocline dwelling CWCs at various sites in the eastern Atlantic Ocean, with intermittently centennial resolution over the last 32 ka. Shallow dwelling CWCs off Angola, located in the South Atlantic, infer a link between the mid‐depth equatorial Atlantic and Southern Ocean. They confirm a 14C drawdown during the Last Glacial Maximum (LGM) and advocate for a consistent Southern Hemisphere radiocarbon aging of upper thermocline waters, as well as strong depth gradients and high variability. Direct comparison with 14C simulations carried out with an Ocean General Circulation Model yield good agreement for Angola. In contrast, the North Atlantic thermocline shows well‐ventilated water with strong variations near the position of today's Azores Front (AF), neither of which are captured by the model. During the Bølling‐Allerød, we confirm the important role of the AF in separating North and South Atlantic thermocline waters and provide further evidence of a 500 year long deep convection interruption within the Younger Dryas (YD). We conclude that the North and South Atlantic thermocline waters were separately acting carbon reservoirs during the LGM and subsequent deglaciation until the modern circulation was established during the YD.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43719517","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
I. Gil, J. McManus, A. Rebotim, Á. Narciso, E. Salgueiro, F. Abrantes
Relatively high opal concentrations are measured in equatorial Atlantic sediments from the most recent deglaciation. To shed light on their causes, seven cores were analyzed for their content of siliceous (diatom, silicoflagellates, radiolarians, phytoliths, and sponge spicules) and calcareous (coccolithophores) microfossils. An early deglacial signal is detected at the time of rising boreal summer insolation ca. 18 ka by the coccolithophores. The surface freshening is likely due to the rain belt associated with the intertropical convergence zone (ITCZ), implying its southward shift relatively to its present‐day average positioning. The diatom assemblages corresponding to the following increase in diatom abundances ca. 15.5 ka suggest the formation of a cold tongue of upwelled water associated with tropical instability waves propagating westward. Such conditions occur at present during boreal summer, when southerly trade winds are intensified, and the ITCZ shifts northward. The presence of the diatom Ethmodiscus rex (Wallich) Hendey and the coccolithophore Florisphera profunda indicates a deep thermocline and nutrient enrichment of the lower photic zone, revealing that Si‐rich southern sourced water (SSW) likely contributed to enhanced primary productivity during this time interval. The discrepancies between the maximum opal concentrations and siliceous marine microfossils records evidence the contribution of freshwater diatoms and phytoliths, indicative of other processes. The definition of the nature of the opal record suggests successive productivity conditions associated with specific atmospheric settings determining the latitudinal ITCZ positioning and the development of oceanic processes; and major oceanic circulation changes permitting the contribution of SSW to marine productivity at this latitude.
{"title":"The Nature of Opal Burial in the Equatorial Atlantic During the Deglaciation","authors":"I. Gil, J. McManus, A. Rebotim, Á. Narciso, E. Salgueiro, F. Abrantes","doi":"10.1029/2022PA004582","DOIUrl":"https://doi.org/10.1029/2022PA004582","url":null,"abstract":"Relatively high opal concentrations are measured in equatorial Atlantic sediments from the most recent deglaciation. To shed light on their causes, seven cores were analyzed for their content of siliceous (diatom, silicoflagellates, radiolarians, phytoliths, and sponge spicules) and calcareous (coccolithophores) microfossils. An early deglacial signal is detected at the time of rising boreal summer insolation ca. 18 ka by the coccolithophores. The surface freshening is likely due to the rain belt associated with the intertropical convergence zone (ITCZ), implying its southward shift relatively to its present‐day average positioning. The diatom assemblages corresponding to the following increase in diatom abundances ca. 15.5 ka suggest the formation of a cold tongue of upwelled water associated with tropical instability waves propagating westward. Such conditions occur at present during boreal summer, when southerly trade winds are intensified, and the ITCZ shifts northward. The presence of the diatom Ethmodiscus rex (Wallich) Hendey and the coccolithophore Florisphera profunda indicates a deep thermocline and nutrient enrichment of the lower photic zone, revealing that Si‐rich southern sourced water (SSW) likely contributed to enhanced primary productivity during this time interval. The discrepancies between the maximum opal concentrations and siliceous marine microfossils records evidence the contribution of freshwater diatoms and phytoliths, indicative of other processes. The definition of the nature of the opal record suggests successive productivity conditions associated with specific atmospheric settings determining the latitudinal ITCZ positioning and the development of oceanic processes; and major oceanic circulation changes permitting the contribution of SSW to marine productivity at this latitude.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44881091","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
F. D. Kelemen, S. Steinig, A. D. de Boer, Jiang Zhu, W. Chan, I. Niezgodzki, D. Hutchinson, G. Knorr, A. Abe‐Ouchi, B. Ahrens
The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP). These simulations target the early Eocene climatic optimum, a geological time period with high CO2 concentrations, analog to the upper range of end‐of‐century CO2 projections. Preindustrial and early Eocene simulations, at a range of CO2 levels are used to quantify the MHT changes in response to both CO2 and non‐CO2 related forcings. We found that atmospheric poleward heat transport increases with CO2, while oceanic poleward heat transport decreases. The non‐CO2 boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO2 concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. Our results show that the changes in the monsoon systems' latent heat transport is a robust feature of CO2 warming, which is in line with the currently observed precipitation increase of present day monsoon systems.
{"title":"Meridional Heat Transport in the DeepMIP Eocene Ensemble: Non‐CO2 and CO2 Effects","authors":"F. D. Kelemen, S. Steinig, A. D. de Boer, Jiang Zhu, W. Chan, I. Niezgodzki, D. Hutchinson, G. Knorr, A. Abe‐Ouchi, B. Ahrens","doi":"10.1029/2022PA004607","DOIUrl":"https://doi.org/10.1029/2022PA004607","url":null,"abstract":"The total meridional heat transport (MHT) is relatively stable across different climates. Nevertheless, the strength of individual processes contributing to the total transport are not stable. Here we investigate the MHT and its main components especially in the atmosphere, in five coupled climate model simulations from the Deep‐Time Model Intercomparison Project (DeepMIP). These simulations target the early Eocene climatic optimum, a geological time period with high CO2 concentrations, analog to the upper range of end‐of‐century CO2 projections. Preindustrial and early Eocene simulations, at a range of CO2 levels are used to quantify the MHT changes in response to both CO2 and non‐CO2 related forcings. We found that atmospheric poleward heat transport increases with CO2, while oceanic poleward heat transport decreases. The non‐CO2 boundary conditions cause more MHT toward the South Pole, mainly through an increase in the southward oceanic heat transport. The changes in paleogeography increase the heat transport via transient eddies at the northern mid‐latitudes in the Eocene. The Eocene Hadley cells do not transport more heat poleward, but due to the warmer atmosphere, especially the northern cell, circulate more heat in the tropics, than today. The monsoon systems' poleward latent heat transport increases with rising CO2 concentrations, but this change is counterweighted by the globally smaller Eocene monsoon area. Our results show that the changes in the monsoon systems' latent heat transport is a robust feature of CO2 warming, which is in line with the currently observed precipitation increase of present day monsoon systems.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42524083","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Arctic forests of the Eocene, which thrived under elevated CO2, a temperate climate, high precipitation and annually extremely different daylengths, represent a quite spectacular no‐analogue habitat of Earth's greenhouse past. The aim of this study was to improve our understanding of the ecophysiology of Arctic broad‐leaved deciduous forests of the Eocene, by analyzing leaf photosynthesis and tree productivity based on gas exchange modeling for two fossil Eocene sites, Svalbard and Ellesmere Island. For this, a single‐leaf photosynthesis model that includes heat transfer and leaf senescence was derived. Environmental conditions were based on available palaeoclimate data and a CO2 level of 800 μmol/mol. Additionally, different light regimes (diffusivity and transmissivity) were considered. With this model, annual photosynthesis was calculated on the basis of annual temperature and day lengths (derived by celestial mechanics). To obtain productivity of a whole deciduous broad‐leaved tree, the single leaf data were then upscaled by a canopy model. The results indicate that productivity was enhanced at both high latitude sites by elevated CO2, temperature of the growing season and high maximum daylength (24 hr) during late spring and early summer. With productivity values about 30%–60% higher as for a mid‐latitude continental European forest, the results indicate a potential for high productivity at the Eocene polar sites which is in the range of extant tropical forests. In contrast to speculations, no evidence for a selective advantage of large leaf size—as shown by various fossil leaves from high latitude sites—could be found.
{"title":"High Productivity at High Latitudes? Photosynthesis and Leaf Ecophysiology in Arctic Forests of the Eocene","authors":"W. Konrad, A. Roth-Nebelsick, C. Traiser","doi":"10.1029/2023PA004685","DOIUrl":"https://doi.org/10.1029/2023PA004685","url":null,"abstract":"The Arctic forests of the Eocene, which thrived under elevated CO2, a temperate climate, high precipitation and annually extremely different daylengths, represent a quite spectacular no‐analogue habitat of Earth's greenhouse past. The aim of this study was to improve our understanding of the ecophysiology of Arctic broad‐leaved deciduous forests of the Eocene, by analyzing leaf photosynthesis and tree productivity based on gas exchange modeling for two fossil Eocene sites, Svalbard and Ellesmere Island. For this, a single‐leaf photosynthesis model that includes heat transfer and leaf senescence was derived. Environmental conditions were based on available palaeoclimate data and a CO2 level of 800 μmol/mol. Additionally, different light regimes (diffusivity and transmissivity) were considered. With this model, annual photosynthesis was calculated on the basis of annual temperature and day lengths (derived by celestial mechanics). To obtain productivity of a whole deciduous broad‐leaved tree, the single leaf data were then upscaled by a canopy model. The results indicate that productivity was enhanced at both high latitude sites by elevated CO2, temperature of the growing season and high maximum daylength (24 hr) during late spring and early summer. With productivity values about 30%–60% higher as for a mid‐latitude continental European forest, the results indicate a potential for high productivity at the Eocene polar sites which is in the range of extant tropical forests. In contrast to speculations, no evidence for a selective advantage of large leaf size—as shown by various fossil leaves from high latitude sites—could be found.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44099836","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ruby Barrett, Monsuru Adebowale, Heather Birch, Jamie D. Wilson, D. Schmidt
Carbonate‐forming organisms play an integral role in the marine inorganic carbon cycle, yet the links between carbonate production and the environment are insufficiently understood. Carbonate production is driven by the abundance of calcifiers and the amount of calcite produced by each individual (their size and weight). Here we investigate how foraminiferal carbonate production changes in the Atlantic, Pacific and Southern Ocean in response to a 4–5°C warming and a 0.3 surface ocean pH reduction during the Palaeocene‐Eocene Thermal Maximum (PETM). To put these local data into a global context, we apply a trait‐based plankton model (ForamEcoGEnIE) to the geologic record for the first time. Our data illustrates negligible change in the assemblage test size and abundance of foraminifers. ForamEcoGEnIE resolves small reductions in size and biomass, but these are short‐lived. The response of foraminifers shows spatial variability linked to a warming‐induced poleward migration and suggested differences in nutrient availability between open‐ocean and shelf locations. Despite low calcite saturation at high latitudes, we reconstruct stable foraminiferal size‐normalized weight. Based on these findings, we postulate that sea surface warming had a greater impact on foraminiferal carbonate production during the PETM than ocean acidification. Changes in the composition of bulk carbonate suggest a higher sensitivity of coccolithophores to environmental change during the PETM than foraminifers.
{"title":"Planktic Foraminiferal Resilience to Environmental Change Associated With the PETM","authors":"Ruby Barrett, Monsuru Adebowale, Heather Birch, Jamie D. Wilson, D. Schmidt","doi":"10.1029/2022PA004534","DOIUrl":"https://doi.org/10.1029/2022PA004534","url":null,"abstract":"Carbonate‐forming organisms play an integral role in the marine inorganic carbon cycle, yet the links between carbonate production and the environment are insufficiently understood. Carbonate production is driven by the abundance of calcifiers and the amount of calcite produced by each individual (their size and weight). Here we investigate how foraminiferal carbonate production changes in the Atlantic, Pacific and Southern Ocean in response to a 4–5°C warming and a 0.3 surface ocean pH reduction during the Palaeocene‐Eocene Thermal Maximum (PETM). To put these local data into a global context, we apply a trait‐based plankton model (ForamEcoGEnIE) to the geologic record for the first time. Our data illustrates negligible change in the assemblage test size and abundance of foraminifers. ForamEcoGEnIE resolves small reductions in size and biomass, but these are short‐lived. The response of foraminifers shows spatial variability linked to a warming‐induced poleward migration and suggested differences in nutrient availability between open‐ocean and shelf locations. Despite low calcite saturation at high latitudes, we reconstruct stable foraminiferal size‐normalized weight. Based on these findings, we postulate that sea surface warming had a greater impact on foraminiferal carbonate production during the PETM than ocean acidification. Changes in the composition of bulk carbonate suggest a higher sensitivity of coccolithophores to environmental change during the PETM than foraminifers.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49382075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Several modes of tropical sea‐surface temperature (SST) variability operate on year‐to‐year (interannual) timescales and profoundly shape seasonal precipitation patterns across adjacent landmasses. Substantial uncertainty remains in addressing how SST variability will become altered under sustained greenhouse warming. Paleoceanographic estimates of changes in variability under past climatic states have emerged as a powerful method to clarify the sensitivity of interannual variability to climate forcing. Several approaches have been developed to investigate interannual SST variability within and beyond the observational period, primarily using marine calcifiers that afford subannual‐resolution sampling plans. Amongst these approaches, geochemical variations in coral skeletons are particularly attractive for their near‐monthly, continuous sampling resolution, and capacity to focus on SST anomalies after removing an annual cycle calculated over many years (represented as geochemical oscillations). Here we briefly review the paleoceanographic pursuit of interannual variability. We additionally highlight recent research documented by Ong et al., (2022, https://doi.org/10.1029/2022PA004483) who demonstrate the utility of Sr/Ca variations in capturing SST variability using a difficult‐to‐sample meandroid coral species, Colpophyllia natans, which is widespread across the Caribbean region and can be used to generate records spanning multiple centuries.
{"title":"Chasing Interannual Marine Paleovariability","authors":"K. Thirumalai, C. Maupin","doi":"10.1029/2023PA004723","DOIUrl":"https://doi.org/10.1029/2023PA004723","url":null,"abstract":"Several modes of tropical sea‐surface temperature (SST) variability operate on year‐to‐year (interannual) timescales and profoundly shape seasonal precipitation patterns across adjacent landmasses. Substantial uncertainty remains in addressing how SST variability will become altered under sustained greenhouse warming. Paleoceanographic estimates of changes in variability under past climatic states have emerged as a powerful method to clarify the sensitivity of interannual variability to climate forcing. Several approaches have been developed to investigate interannual SST variability within and beyond the observational period, primarily using marine calcifiers that afford subannual‐resolution sampling plans. Amongst these approaches, geochemical variations in coral skeletons are particularly attractive for their near‐monthly, continuous sampling resolution, and capacity to focus on SST anomalies after removing an annual cycle calculated over many years (represented as geochemical oscillations). Here we briefly review the paleoceanographic pursuit of interannual variability. We additionally highlight recent research documented by Ong et al., (2022, https://doi.org/10.1029/2022PA004483) who demonstrate the utility of Sr/Ca variations in capturing SST variability using a difficult‐to‐sample meandroid coral species, Colpophyllia natans, which is widespread across the Caribbean region and can be used to generate records spanning multiple centuries.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43226362","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The quantitative and objective characterization of dissolution intensity in fossil planktonic foraminiferal shells could be used to reconstruct past changes in bottom water carbonate ion concentration. Among proxies measuring the degree of dissolution of planktonic foraminiferal shells, X‐ray micro‐Computed Tomography (CT) based characterization of apparent shell density appears to have good potential to facilitate quantitative reconstruction of carbonate chemistry. However, unlike the well‐established benthic foraminiferal B/Ca ratio‐based proxy, only a regional calibration of the CT‐based proxy exists based on a limited number of data points covering mainly low‐saturation state waters. Here we determined by CT‐based proxy the shell dissolution intensity of planktonic foraminifera Globigerina bulloides, Globorotalia inflata, Globigerinoides ruber, and Trilobatus sacculifer from a collection of core top samples in the Southern Atlantic covering higher saturation states and assessed the reliability of CT‐based proxy. We observed that the CT‐based proxy is generally controlled by deep‐water Δ[ CO32– ${{mathrm{C}mathrm{O}}_{3}}^{2mbox{--}}$ ] like the B/Ca proxy, but its effective range of Δ[ CO32– ${{mathrm{C}mathrm{O}}_{3}}^{2mbox{--}}$ ] is between −20 and 10 µmolkg−1. In this range, the CT‐based proxy appears directly and strongly related to deep‐water Δ[ CO32– ${{mathrm{C}mathrm{O}}_{3}}^{2mbox{--}}$ ], whereas we note that in some settings, there appears to be a secondary influence on B/Ca which we suggest may be due to elevated alkalinity from carbonate dissolution in sediments. On the other hand, the CT‐based proxy is affected by supralysoclinal dissolution in areas with high productivity. Like the B/Ca proxy, the CT‐based proxy requires species‐specific calibration, but the effect of species‐specific shell difference in susceptibility to dissolution on the proxy is small.
{"title":"Development of a Deep‐Water Carbonate Ion Concentration Proxy Based on Preservation of Planktonic Foraminifera Shells Quantified by X‐Ray CT Scanning","authors":"S. Iwasaki, K. Kimoto, M. Kučera","doi":"10.1029/2022PA004601","DOIUrl":"https://doi.org/10.1029/2022PA004601","url":null,"abstract":"The quantitative and objective characterization of dissolution intensity in fossil planktonic foraminiferal shells could be used to reconstruct past changes in bottom water carbonate ion concentration. Among proxies measuring the degree of dissolution of planktonic foraminiferal shells, X‐ray micro‐Computed Tomography (CT) based characterization of apparent shell density appears to have good potential to facilitate quantitative reconstruction of carbonate chemistry. However, unlike the well‐established benthic foraminiferal B/Ca ratio‐based proxy, only a regional calibration of the CT‐based proxy exists based on a limited number of data points covering mainly low‐saturation state waters. Here we determined by CT‐based proxy the shell dissolution intensity of planktonic foraminifera Globigerina bulloides, Globorotalia inflata, Globigerinoides ruber, and Trilobatus sacculifer from a collection of core top samples in the Southern Atlantic covering higher saturation states and assessed the reliability of CT‐based proxy. We observed that the CT‐based proxy is generally controlled by deep‐water Δ[ CO32– ${{mathrm{C}mathrm{O}}_{3}}^{2mbox{--}}$ ] like the B/Ca proxy, but its effective range of Δ[ CO32– ${{mathrm{C}mathrm{O}}_{3}}^{2mbox{--}}$ ] is between −20 and 10 µmolkg−1. In this range, the CT‐based proxy appears directly and strongly related to deep‐water Δ[ CO32– ${{mathrm{C}mathrm{O}}_{3}}^{2mbox{--}}$ ], whereas we note that in some settings, there appears to be a secondary influence on B/Ca which we suggest may be due to elevated alkalinity from carbonate dissolution in sediments. On the other hand, the CT‐based proxy is affected by supralysoclinal dissolution in areas with high productivity. Like the B/Ca proxy, the CT‐based proxy requires species‐specific calibration, but the effect of species‐specific shell difference in susceptibility to dissolution on the proxy is small.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":"17 2","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41263630","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
At the Eocene‐Oligocene Transition (EOT), approximately 34 million years ago, Earth abruptly transitioned to a climate state sufficiently cool for Antarctica to sustain large ice sheets for the first time in tens to hundreds of millions of years. Oxygen isotope records from deep‐sea benthic foraminifera (δ18Ob) provide the foundation of our understanding of this pivot point in Cenozoic climate history. A deeper insight, however, is hindered by the paucity of independent deep‐sea temperature reconstructions and the ongoing challenge of deconvolving the temperature and continental ice volume signals embedded in δ18Ob records. Here we present records of deep‐sea temperature change from the eastern equatorial Pacific for the EOT using clumped isotope thermometry, which permits explicit temperature reconstructions independent of seawater chemistry and continental ice volume. Our records suggest that the deep Pacific Ocean cooled markedly at the EOT by 4.7 ± 0.9°C. This decrease in temperature represents the first direct and robust evidence of deep‐sea cooling associated with the inception of major Cenozoic glaciation. However, our data also indicate that this major cooling of the deep Pacific Ocean at the EOT was short‐lived (∼200 kyrs), with temperatures rebounding to values close to pre‐EOT levels by 33.6 Ma. Our calculated record of seawater δ18O suggests that this rebound in ocean temperature occurred despite the continued presence of a large‐scale Antarctic ice sheet. This finding suggests a degree of decoupling between deep ocean temperatures in the eastern equatorial Pacific Ocean and the behavior of the newly established Antarctic ice sheet.
{"title":"Transient Deep Ocean Cooling in the Eastern Equatorial Pacific Ocean at the Eocene‐Oligocene Transition","authors":"V. Taylor, P. Wilson, S. Bohaty, A. N. Meckler","doi":"10.1029/2023PA004650","DOIUrl":"https://doi.org/10.1029/2023PA004650","url":null,"abstract":"At the Eocene‐Oligocene Transition (EOT), approximately 34 million years ago, Earth abruptly transitioned to a climate state sufficiently cool for Antarctica to sustain large ice sheets for the first time in tens to hundreds of millions of years. Oxygen isotope records from deep‐sea benthic foraminifera (δ18Ob) provide the foundation of our understanding of this pivot point in Cenozoic climate history. A deeper insight, however, is hindered by the paucity of independent deep‐sea temperature reconstructions and the ongoing challenge of deconvolving the temperature and continental ice volume signals embedded in δ18Ob records. Here we present records of deep‐sea temperature change from the eastern equatorial Pacific for the EOT using clumped isotope thermometry, which permits explicit temperature reconstructions independent of seawater chemistry and continental ice volume. Our records suggest that the deep Pacific Ocean cooled markedly at the EOT by 4.7 ± 0.9°C. This decrease in temperature represents the first direct and robust evidence of deep‐sea cooling associated with the inception of major Cenozoic glaciation. However, our data also indicate that this major cooling of the deep Pacific Ocean at the EOT was short‐lived (∼200 kyrs), with temperatures rebounding to values close to pre‐EOT levels by 33.6 Ma. Our calculated record of seawater δ18O suggests that this rebound in ocean temperature occurred despite the continued presence of a large‐scale Antarctic ice sheet. This finding suggests a degree of decoupling between deep ocean temperatures in the eastern equatorial Pacific Ocean and the behavior of the newly established Antarctic ice sheet.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48256618","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Whereas changes in pore water chemistry are known to impact carbonate sediment geochemistry, little is known about the impact of long unconformities on carbonate alteration. IODP Site 378‐U1553 on the southern Campbell Plateau, with a 26‐million‐year, erosional unconformity, provides a key location for examining the impact of long‐term unconformities on sub‐surface chemistry and carbonate archives. This study examined 76 interstitial water samples for sulfate sulfur and oxygen isotopes, as well as 28 bulk carbonate samples for carbonate associated sulfate (CAS) sulfur isotopes, to quantify the effects of this unconformity on the sub‐surface redox chemistry of the Site. The current state of the system suggests limited influence of redox processes on the CAS archive. Manganese reduction reaches 30 mbsf, with a decrease in manganese reduction between 20 and 30 mbsf. Below 30 mbsf, the system transitions to iron reduction to a depth of approximately 140 mbsf where sulfate reduction begins. Dissolved sulfate sulfur and oxygen isotope values suggest repeated oxygenation of sulfides. The CAS record from the Site deviates from previously published seawater values. However, the lack of a relationship between the dissolved sulfate and CAS records suggests most of the alteration of the CAS record likely occurred before the unconformity when the carbonate sediments were more reactive. This further supports the CAS record as a relatively robust archive, withstanding most post‐depositional mechanisms of carbonate alteration.
{"title":"Changing Sub‐Surface Chemistry Resulting From a 26‐Million‐Year Unconformity: Porewater Chemistry From IODP Site U1553 in the South Pacific","authors":"A. Reis, V. Fichtner, A. Erhardt, A. Reis","doi":"10.1029/2022PA004561","DOIUrl":"https://doi.org/10.1029/2022PA004561","url":null,"abstract":"Whereas changes in pore water chemistry are known to impact carbonate sediment geochemistry, little is known about the impact of long unconformities on carbonate alteration. IODP Site 378‐U1553 on the southern Campbell Plateau, with a 26‐million‐year, erosional unconformity, provides a key location for examining the impact of long‐term unconformities on sub‐surface chemistry and carbonate archives. This study examined 76 interstitial water samples for sulfate sulfur and oxygen isotopes, as well as 28 bulk carbonate samples for carbonate associated sulfate (CAS) sulfur isotopes, to quantify the effects of this unconformity on the sub‐surface redox chemistry of the Site. The current state of the system suggests limited influence of redox processes on the CAS archive. Manganese reduction reaches 30 mbsf, with a decrease in manganese reduction between 20 and 30 mbsf. Below 30 mbsf, the system transitions to iron reduction to a depth of approximately 140 mbsf where sulfate reduction begins. Dissolved sulfate sulfur and oxygen isotope values suggest repeated oxygenation of sulfides. The CAS record from the Site deviates from previously published seawater values. However, the lack of a relationship between the dissolved sulfate and CAS records suggests most of the alteration of the CAS record likely occurred before the unconformity when the carbonate sediments were more reactive. This further supports the CAS record as a relatively robust archive, withstanding most post‐depositional mechanisms of carbonate alteration.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":" ","pages":""},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45292865","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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