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":null,"pages":null},"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}
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":null,"pages":null},"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}
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":null,"pages":null},"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}
Jiawei Da, D. Breecker, Tao Li, Gaojun Li, Huayu Lu, Junfeng Ji
Understanding the monsoonal climate over East Asia during the warm Pliocene, the closest analog of the future warm climate, could better inform us of the regional hydrological responses to global climate change. However, the variations and controlling mechanisms of the regional hydrology during this warm period are not determined due to discrepancies among different proxy‐derived records. Here we apply a multiproxy approach based on the geochemistry of calcite nodules from a Red Clay sequence located on the southern edge of the Chinese Loess Plateau. Both the trace metal/Ca ratios and the carbon and oxygen isotopic compositions of calcite nodules show low values during 5.4–4.1 Ma and increased during 4.1–3.3 Ma, together indicating a humid climate during the early Pliocene, the onset of drying starting at ∼4.1 Ma and further intensification at 3.6 Ma. The timings of these hydrological transitions are consistent with global temperature changes, underlining the crucial role of meridional thermal gradient in shaping the regional hydroclimate over East Asia by modulating the strength and position of the East Asian summer monsoon.
{"title":"A Humid East Asia During the Early Pliocene Indicated by Calcite Nodules From the Chinese Loess Plateau","authors":"Jiawei Da, D. Breecker, Tao Li, Gaojun Li, Huayu Lu, Junfeng Ji","doi":"10.1029/2023PA004615","DOIUrl":"https://doi.org/10.1029/2023PA004615","url":null,"abstract":"Understanding the monsoonal climate over East Asia during the warm Pliocene, the closest analog of the future warm climate, could better inform us of the regional hydrological responses to global climate change. However, the variations and controlling mechanisms of the regional hydrology during this warm period are not determined due to discrepancies among different proxy‐derived records. Here we apply a multiproxy approach based on the geochemistry of calcite nodules from a Red Clay sequence located on the southern edge of the Chinese Loess Plateau. Both the trace metal/Ca ratios and the carbon and oxygen isotopic compositions of calcite nodules show low values during 5.4–4.1 Ma and increased during 4.1–3.3 Ma, together indicating a humid climate during the early Pliocene, the onset of drying starting at ∼4.1 Ma and further intensification at 3.6 Ma. The timings of these hydrological transitions are consistent with global temperature changes, underlining the crucial role of meridional thermal gradient in shaping the regional hydroclimate over East Asia by modulating the strength and position of the East Asian summer monsoon.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44990537","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":null,"pages":null},"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}
M. Galochkina, A. Cohen, D. Oppo, N. Mollica, F. Horton
Understanding climate change at the spatiotemporal scales necessary to improve climate projections requires proxy records that complement sparse and often contradictory observational temperature data sets. Massive long‐lived corals have tremendous potential in this regard, continuously recording information about ocean conditions as they grow. Nevertheless, extracting accurate ocean temperatures from corals is challenging because factors other than temperature influence skeletal chemistry. Here, we tested the ability of the coral Sr‐U thermometer to accurately capture annual sea surface temperatures (SSTs) in the subtropical Atlantic, where year‐to‐year temperatures vary by ∼1°C. Using laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS), we generated sufficient U/Ca – Sr/Ca pairs from a slow‐growing (1−2 mm/yr) Siderastrea siderea coral to calculate annual Sr‐U values. With the fine‐scale spatial resolution attained using the laser, skeleton accreted during both fast and slow growing times of the year was represented in our sampling. The resulting 30‐year‐long Sr‐U record tracked the amplitude and timing of annual SST to within ±0.2°C of observations (r = −0.71), whereas the Sr/Ca record did not (r = 0.23). Furthermore, Sr‐U corrected for Sr/Ca offsets among adjacent skeletal elements approximately 1 mm apart. These offsets are equivalent to differences of 2–3°C if typical Sr/Ca–SST calibrations are applied. Our observations indicate that Sr‐U can accurately constrain decadal‐to‐multidecadal variability and secular SST trends in regions where this information is urgently needed.
{"title":"Coral Sr‐U Thermometry Tracks Ocean Temperature and Reconciles Sr/Ca Discrepancies Caused by Rayleigh Fractionation","authors":"M. Galochkina, A. Cohen, D. Oppo, N. Mollica, F. Horton","doi":"10.1029/2022PA004541","DOIUrl":"https://doi.org/10.1029/2022PA004541","url":null,"abstract":"Understanding climate change at the spatiotemporal scales necessary to improve climate projections requires proxy records that complement sparse and often contradictory observational temperature data sets. Massive long‐lived corals have tremendous potential in this regard, continuously recording information about ocean conditions as they grow. Nevertheless, extracting accurate ocean temperatures from corals is challenging because factors other than temperature influence skeletal chemistry. Here, we tested the ability of the coral Sr‐U thermometer to accurately capture annual sea surface temperatures (SSTs) in the subtropical Atlantic, where year‐to‐year temperatures vary by ∼1°C. Using laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS), we generated sufficient U/Ca – Sr/Ca pairs from a slow‐growing (1−2 mm/yr) Siderastrea siderea coral to calculate annual Sr‐U values. With the fine‐scale spatial resolution attained using the laser, skeleton accreted during both fast and slow growing times of the year was represented in our sampling. The resulting 30‐year‐long Sr‐U record tracked the amplitude and timing of annual SST to within ±0.2°C of observations (r = −0.71), whereas the Sr/Ca record did not (r = 0.23). Furthermore, Sr‐U corrected for Sr/Ca offsets among adjacent skeletal elements approximately 1 mm apart. These offsets are equivalent to differences of 2–3°C if typical Sr/Ca–SST calibrations are applied. Our observations indicate that Sr‐U can accurately constrain decadal‐to‐multidecadal variability and secular SST trends in regions where this information is urgently needed.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44501131","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}
Although previous studies have shown that the paleoenvironment and geological processes contributed to the organic matter accumulation (OMA) and carbon isotope excursions (CIEs) during the late Ordovician–early Silurian, the dominated controlling factor for the OMA and the origins of CIEs still remains unclear due to complex interaction between various paleoenvironmental factors and geological processes. Therefore, based on the elemental geochemistry of the Wufeng–Longmaxi Formation shales in the upper Yangtze Platform, we analyzed the late Ordovician–early Silurian paleoenvironment and related geological processes, and further explored the origin of the OMA and CIEs. As a result, the Wufeng–Longmaxi Formation shale was divided into four stages. During Stage 1 (late Katian, ∼447.62–444.50 Ma), local tectonic and volcanic activities controlled the paleoproductivity and redox conditions, facilitating the OMA. By contrast, the productivity of the surface water and the anoxic bottom water were mainly controlled by the global climate after the Hirnantian glaciation, which contributed to the OMA during Stage 2 (early Rhuddanian, ∼444.50–441.00 Ma). The decreasing sea level and rapid uplifting of Xuefeng and Qianzhong Uplifts resulted in the organic matter depletion during Stages 3 (late Rhuddanian, ∼441.00–440.80 Ma) and 4 (Aeronian, ∼440.80–439.21 Ma). Besides, the release of 12C–enriched carbon reservoirs triggered by volcanic activities and 13C–enrichment caused by the OMA regulated the carbon cycling: the negative CIE may be the result of light carbon emissions, such as the reactivated organic matter and mantle derived carbon, and the Hirnantian CIE event is jointly controlled by the weakened carbon emission effect and the OMA.
{"title":"Influence of the Late Ordovician‐Early Silurian Paleoenvironment and Related Geological Processes on the Organic Matter Accumulation and Carbon Isotope Excursion","authors":"Pengyuan Zhang, Yongli Wang, Zhifu Wei, Gen-xu Wang, Ting Zhang, Wei He, Xueyun Ma, He Ma, Jingyi Wei, Chenxi Zhu","doi":"10.1029/2023PA004628","DOIUrl":"https://doi.org/10.1029/2023PA004628","url":null,"abstract":"Although previous studies have shown that the paleoenvironment and geological processes contributed to the organic matter accumulation (OMA) and carbon isotope excursions (CIEs) during the late Ordovician–early Silurian, the dominated controlling factor for the OMA and the origins of CIEs still remains unclear due to complex interaction between various paleoenvironmental factors and geological processes. Therefore, based on the elemental geochemistry of the Wufeng–Longmaxi Formation shales in the upper Yangtze Platform, we analyzed the late Ordovician–early Silurian paleoenvironment and related geological processes, and further explored the origin of the OMA and CIEs. As a result, the Wufeng–Longmaxi Formation shale was divided into four stages. During Stage 1 (late Katian, ∼447.62–444.50 Ma), local tectonic and volcanic activities controlled the paleoproductivity and redox conditions, facilitating the OMA. By contrast, the productivity of the surface water and the anoxic bottom water were mainly controlled by the global climate after the Hirnantian glaciation, which contributed to the OMA during Stage 2 (early Rhuddanian, ∼444.50–441.00 Ma). The decreasing sea level and rapid uplifting of Xuefeng and Qianzhong Uplifts resulted in the organic matter depletion during Stages 3 (late Rhuddanian, ∼441.00–440.80 Ma) and 4 (Aeronian, ∼440.80–439.21 Ma). Besides, the release of 12C–enriched carbon reservoirs triggered by volcanic activities and 13C–enrichment caused by the OMA regulated the carbon cycling: the negative CIE may be the result of light carbon emissions, such as the reactivated organic matter and mantle derived carbon, and the Hirnantian CIE event is jointly controlled by the weakened carbon emission effect and the OMA.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42231078","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}
K. Yasukawa, E. Tanaka, T. Miyazaki, B. Vaglarov, Q. Chang, K. Nakamura, J. Ohta, K. Fujinaga, H. Iwamori, Y. Kato
Pelagic clay constitutes massive and apparently uniform lithology that limits the stratigraphic correlation between neighboring sediment core samples. Recent studies on the pelagic clay in the western North Pacific Ocean demonstrated that the bulk chemical composition of sediments constitutes multielemental chemostratigraphy, deducing the correlation between visibly featureless pelagic clay layers across several cores. However, this heuristic approach utilized only a few elements. Therefore, this study employed multivariate statistical techniques, including k‐means cluster analysis, to analyze the chemical composition data set of 1,646 samples × 41 elements of the western North Pacific pelagic clay. The pelagic clay was classified into 10 clusters systematically aligned from the seafloor to the depth in a specific order, constituting stratigraphic units that reflected the high‐dimensional geochemical features of these 41 elements. This finding strongly supports the statistical robustness of the latent chemostratigraphy in the western North Pacific pelagic clay. Additionally, we performed Sr–Nd–Pb isotope analyses of the detrital silicate fractions of the centroid samples representing each cluster. The multi‐isotopic features of the detrital fraction varied from a mixture of North American and Asian dusts to a predominance of Asian dust superimposed by volcanic inputs. This secular variation in the matrix components is attributable to the northwestward motion of the Pacific Plate and the time‐varying influence of arc volcanism throughout sedimentary history. The proposed integrated approach of multivariate statistical and isotopic analyses effectively extracted the essential information hidden in the pelagic clay, which may postulate a new protocol for paleoceanographic reconstructions targeting the pelagic realm.
{"title":"High‐Dimensional Chemostratigraphy of Pelagic Clay in the Western North Pacific Ocean Revealed via an Unsupervised Clustering Approach","authors":"K. Yasukawa, E. Tanaka, T. Miyazaki, B. Vaglarov, Q. Chang, K. Nakamura, J. Ohta, K. Fujinaga, H. Iwamori, Y. Kato","doi":"10.1029/2023PA004644","DOIUrl":"https://doi.org/10.1029/2023PA004644","url":null,"abstract":"Pelagic clay constitutes massive and apparently uniform lithology that limits the stratigraphic correlation between neighboring sediment core samples. Recent studies on the pelagic clay in the western North Pacific Ocean demonstrated that the bulk chemical composition of sediments constitutes multielemental chemostratigraphy, deducing the correlation between visibly featureless pelagic clay layers across several cores. However, this heuristic approach utilized only a few elements. Therefore, this study employed multivariate statistical techniques, including k‐means cluster analysis, to analyze the chemical composition data set of 1,646 samples × 41 elements of the western North Pacific pelagic clay. The pelagic clay was classified into 10 clusters systematically aligned from the seafloor to the depth in a specific order, constituting stratigraphic units that reflected the high‐dimensional geochemical features of these 41 elements. This finding strongly supports the statistical robustness of the latent chemostratigraphy in the western North Pacific pelagic clay. Additionally, we performed Sr–Nd–Pb isotope analyses of the detrital silicate fractions of the centroid samples representing each cluster. The multi‐isotopic features of the detrital fraction varied from a mixture of North American and Asian dusts to a predominance of Asian dust superimposed by volcanic inputs. This secular variation in the matrix components is attributable to the northwestward motion of the Pacific Plate and the time‐varying influence of arc volcanism throughout sedimentary history. The proposed integrated approach of multivariate statistical and isotopic analyses effectively extracted the essential information hidden in the pelagic clay, which may postulate a new protocol for paleoceanographic reconstructions targeting the pelagic realm.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43015778","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 marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation.
{"title":"Dynamics of the Marine Dissolved Organic Carbon Reservoir in Glacial Climate Simulations: The Importance of Biological Production","authors":"Maya D. Gilchrist, Katsumi Matsumoto","doi":"10.1029/2022PA004522","DOIUrl":"https://doi.org/10.1029/2022PA004522","url":null,"abstract":"The marine dissolved organic carbon (DOC) reservoir rivals the atmospheric carbon inventory in size. Recent work has suggested that the size of the DOC reservoir may respond to variations in sea temperature and global overturning circulation strength. Moreover, mobilization of marine DOC has been implicated in paleoclimate events including Cryogenian glaciation and Eocene hyperthermals. Despite these suggestions, the dynamics of the marine DOC reservoir are poorly understood, and previous carbon cycle modeling has generally assumed this reservoir to be static. In this study, we utilize an Earth system model of intermediate complexity to assess the response of the marine DOC reservoir to various glacial boundary conditions. Our results indicate that the marine DOC reservoir is responsive to glacial perturbations and may shrink or expand on the order of 10–100 Pg C. In contrast to recent studies that emphasize the importance of DOC degradation in driving the mobility of DOC reservoir, our study indicates the importance of DOC production. In the experiment under full glacial boundary conditions, for example, a 19% drop in net primary production leads to an 81 Pg C reduction in the DOC pool, without which the atmospheric CO2 concentration would have been lower by approximately 38 ppm by dissolved inorganic carbon changes alone. Thus, DOC reservoir variability is necessary to fully account for the simulated changes in atmospheric CO2 concentration. Our findings based on glacial experiments are corroborated in a different set of simulations using freshwater flux to induce weakening of the Atlantic meridional overturning circulation.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46185272","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}
P. Martínez-Sosa, J. Tierney, L. Pérez‐Angel, I. Stefanescu, Jingjing Guo, F. Kirkels, J. Sepúlveda, F. Peterse, B. Shuman, A. Reyes
Glycerol dialkyl glycerol tetraethers (GDGTs), both archaeal isoprenoid GDGTs (isoGDGTs) and bacterial branched GDGTs (brGDGTs), have been used in paleoclimate studies to reconstruct environmental conditions. Since GDGTs are produced in many types of environments, their relative abundances also depend on the depositional setting. This suggests that the distribution of GDGTs also preserves useful information that can be used more broadly to infer these depositional environments in the geological past. Here, we combined existing iso‐ and brGDGT relative abundance data with newly analyzed samples to generate a database of 1,153 samples from several modern sedimentary settings. We observed a robust relationship between the depositional environment and the relative abundances of GDGTs in our samples. This data set was used to train and test the Branched and isoGDGT Machine learning Classification (BIGMaC) algorithm, which identifies the environment a sample comes from based on the distribution of GDGTs with high precision and recall (F1 = 0.95). We tested the model on the sedimentary record from the Giraffe kimberlite pipe, an Eocene maar in subantarctic Canada, and found that the BIGMaC reconstruction agrees with independent stratigraphic and palynological information, provides new information about the paleoenvironment of this site, and helps improve its paleotemperature reconstruction. In contrast, we also include an example from the PETM‐aged Cobham lignite as a cautionary example that illustrates the limitations of the algorithm. We propose that in cases where paleoenvironments are unknown or are changing, BIGMaC can be applied in concert with other proxies to generate more refined paleoclimate records.
{"title":"Development and Application of the Branched and Isoprenoid GDGT Machine Learning Classification Algorithm (BIGMaC) for Paleoenvironmental Reconstruction","authors":"P. Martínez-Sosa, J. Tierney, L. Pérez‐Angel, I. Stefanescu, Jingjing Guo, F. Kirkels, J. Sepúlveda, F. Peterse, B. Shuman, A. Reyes","doi":"10.1029/2023PA004611","DOIUrl":"https://doi.org/10.1029/2023PA004611","url":null,"abstract":"Glycerol dialkyl glycerol tetraethers (GDGTs), both archaeal isoprenoid GDGTs (isoGDGTs) and bacterial branched GDGTs (brGDGTs), have been used in paleoclimate studies to reconstruct environmental conditions. Since GDGTs are produced in many types of environments, their relative abundances also depend on the depositional setting. This suggests that the distribution of GDGTs also preserves useful information that can be used more broadly to infer these depositional environments in the geological past. Here, we combined existing iso‐ and brGDGT relative abundance data with newly analyzed samples to generate a database of 1,153 samples from several modern sedimentary settings. We observed a robust relationship between the depositional environment and the relative abundances of GDGTs in our samples. This data set was used to train and test the Branched and isoGDGT Machine learning Classification (BIGMaC) algorithm, which identifies the environment a sample comes from based on the distribution of GDGTs with high precision and recall (F1 = 0.95). We tested the model on the sedimentary record from the Giraffe kimberlite pipe, an Eocene maar in subantarctic Canada, and found that the BIGMaC reconstruction agrees with independent stratigraphic and palynological information, provides new information about the paleoenvironment of this site, and helps improve its paleotemperature reconstruction. In contrast, we also include an example from the PETM‐aged Cobham lignite as a cautionary example that illustrates the limitations of the algorithm. We propose that in cases where paleoenvironments are unknown or are changing, BIGMaC can be applied in concert with other proxies to generate more refined paleoclimate records.","PeriodicalId":54239,"journal":{"name":"Paleoceanography and Paleoclimatology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2023-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48127108","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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