Paleoceanography and Paleoclimatology

Journal: Paleoceanography and Paleoclimatology

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Wiley

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2572-4525
2572-4517

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Publications 1 - 10 of 17

Carbon Isotopic Fractionation of Alkenones and Gephyrocapsa Coccoliths Over the Late Quaternary (Marine Isotope Stages 12–9) Glacial-Interglacial Cycles at the Western Tropical Atlantic
González-Lanchas, Alba; Hernández-Almeida, Iván; Flores, José A.; et al. (2021)
Paleoceanography and Paleoclimatology
The sensitivity of coccolithophores to changing CO2 and its role modulating cellular photosynthetic carbon isotopic fractionation (εp) is crucial to understand the future adaptation of these organisms to higher CO2 world and to assess the reliability of εp for past CO2 estimation. Here, we present εp measured on natural fossil samples across the glacial-interglacial (G-I) CO2 variations of marine isotope stages 12 to 9 interval (454–334 ka) at the western tropical Atlantic Ocean Drilling Program Site 925 together with a set of organic and inorganic geochemical, micropaleontological and morphometrical data from Gephyrocapsa coccoliths in the same samples. The ∼2‰ variation in εp is significantly correlated with the CO2[aq] concentrations calculated from assumption of air-sea equilibrium with measured ice core pCO2 concentrations. The sensitivity of εp to CO2[aq] is similar to that derived from a multiple regression model of culture observations and is not well simulated with the classical purely diffusive model of algal CO2 acquisition. The measured range of Gephyrocapsa cell sizes is insufficient to explain the non-CO2 effects on εp at this location, via either direct size effect or growth rate correlated to cell size. Primary productivity, potentially triggered by shifting growth rates and light levels, may also affect εp. Proposed productivity proxies % Florisphaera profunda and the ratio between the C37 to C38.et alkenone (C37/C38.et ratio) both correlates modestly with the non-CO2 effects on εp. When the observed G-I εp to CO2 sensitivity at this site is used to estimate pCO2 from εp since the Miocene, the inferred pCO2 declines are larger in amplitude compared to that calculated from a theoretical εp diffusive model. We find that oxygen and carbon stable isotope vital effects in the near monogeneric-separated Gephyrocapsa coccoliths (respectively Δδ18OGephyrocapsa–Trilobatus sacculifer and εcoccolith) are coupled through the time series, but the origins of these vital effects are not readily explained by existing models.
Poleward Shift in the Southern Hemisphere Westerly Winds Synchronous With the Deglacial Rise in CO2
Gray, William R.; de Lavergne, Casimir; Jnglin Wills, Robert; et al. (2023)
Paleoceanography and Paleoclimatology
The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesized to play a key role in regulating atmospheric CO2 over glacial-interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ18O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9–7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (20 ka) relative to the mid-Holocene (6.5 ka). Climate models from the Palaeoclimate Modeling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO2 (R2 = 0.98). Experiments with a 0.25° resolution ocean-sea-ice-carbon model suggest that shifting the westerlies equatorward reduces the overturning rate of the ocean below 2 km depth, leading to a suppression of CO2 outgassing from the polar Southern Ocean. Our results support a role for the westerly winds in driving the deglacial CO2 rise, and suggest outgassing of natural CO2 from the Southern Ocean is likely to increase as the westerlies shift poleward due to anthropogenic warming.
Mid‐ to Late Holocene Contraction of the Intertropical Convergence Zone Over Northeastern South America
Chiessi, Cristiano; Mulitza, Stefan; Taniguchi, N. K.; et al. (2021)
Paleoceanography and Paleoclimatology
Modern precipitation over northeastern (NE) South America is strongly controlled by the seasonal meridional migration of the Intertropical Convergence Zone (ITCZ). Ample evidence from the Northern Hemisphere suggests a mid- to late Holocene southward migration of the ITCZ. Such a shift would be expected to increase precipitation over semi-arid northern NE Brazil (Southern Hemisphere). However, the most robust precipitation record from northern NE Brazil shows a drying trend throughout the Holocene. Here, we address this issue presenting a high-temporal resolution reconstruction of precipitation over northern NE Brazil based on data from a marine sediment core, together with analyses of mid- and late Holocene simulations performed with the fully coupled climate model FGOALS-s2. Both, our data and the climate model simulations show a decrease in precipitation over northern NE Brazil from the mid- to the late Holocene. The model outputs further indicate a latitudinal contraction of the seasonal migration range of the ITCZ that, together with an intensification of the regional Walker circulation, were responsible for the mid- to late Holocene changes in precipitation over NE South America. Our results reconcile apparently conflicting precipitation records and climate mechanisms used to explain changes in precipitation over NE South America.
A Continuous Record of Central Tropical Pacific Climate Since the Midnineteenth Century Reconstructed From Fanning and Palmyra Island Corals: A Case Study in Coral Data Reanalysis
Sanchez, Sara C.; Westphal, Niko; Haug, Gerald H.; et al. (2020)
Paleoceanography and Paleoclimatology
Accurate estimation of central tropical Pacific (CTP) climate variability on interannual to centennial time scales is required for robust projections of future global climate trends. Here we outline an approach that blends instrumental and coral proxy observations to yield a continuous, monthly resolved record of climate evolution in the CTP spanning the past 160 years. We concatenate coral oxygen isotope (δ18O) records from multiple living and fossil corals collected from Fanning Island (4°N, 160°W) and Palmyra Island (5°N; 162°W) located in the heart of the El Niño–Southern Oscillation. We use the regularized expectation maximization (RegEM) method to impute missing data across short gaps of 5 to 23 years within and beyond individual coral records. The resulting monthly resolved Fanning/Palmyra Island climate record spans continuously from 1863 to 2016 and provides an example of how extended time series can be built from shorter coral segments. The extended record highlights the strong trend toward warmer and wetter mean conditions in late twentieth century, in agreement with the majority of climate model hindcast simulations. The continuous reconstruction also enables a direct comparison of four exceptionally strong El Niño events (1877–1878, 1940–1941, 1997–1998, and 2015–2016). Three of these very strong El Niño events in the CTP featured a precursor warm event in the prior year and that may have favored the development of a strong El Niño event.
Evolution of Sea Surface Temperature in the Southern Mid-latitudes From Late Oligocene Through Early Miocene
Guitián, José; Stoll, Heather M. (2021)
Paleoceanography and Paleoclimatology
Large Antarctic ice volume changes characterized the middle to Late Oligocene and the first million years of climate evolution during the Miocene. However, the sea surface temperature (SST) evolution over this period remains poorly constrained, as only a few records from contrasting proxies are available. In this study, we present a long-term alkenone-derived SST record from sediments drilled by the Ocean Drilling Program (ODP) at Site 1168 in the west Tasmanian Sea spanning 29.8 Ma to 16.7 Ma. The SST record highlight that the long-term warming in the Late Oligocene linked to the end of the Middle Oligocene Glacial Interval can be recognized also at mid-to-high latitudes of the Southern Hemisphere. Warmer average temperatures (25.5°C) characterize the period from 24.6 to 22 Ma; average temperatures then decrease by 1°C–2°C into the Miocene and stabilize by 20.1 Ma. The reconstructed temperatures are highly variable in the warm Late Oligocene waters, and more stable and slightly colder in the Early to Middle Miocene. We confirm that this temperature trend is not an artifact of the latitudinal drift of the site, as the temperature anomaly relative to the modern water temperature at the paleolocation confirms the SST trends of the Oligocene. This is the first alkenone-derived continuous record to reproduce the long-term Oligocene climate trend previously interpreted from the benthic δ18O, which recorded a warming and/or reduction in ice volume from the Middle Oligocene Glacial Interval through the latest Oligocene.
Deep Thermohaline Circulation Across the Closure of the Central American Seaway
Öğretmen, N.; Schiebel, R.; Jochum, K. P.; et al. (2020)
Paleoceanography and Paleoclimatology
The closure of the Central American Seaway (CAS) resulted in changes of ocean-climate dynamics since the late Miocene following the uplift of northern Andes. Reconstructing the bottom-water temperatures (BWTs) of the Caribbean Sea illustrates feedbacks of the closure on the ocean-climate system including deep-water dynamics of the Caribbean Sea. Here, Mg/Ca-derived BWTs of the Plio-Pleistocene Caribbean Sea from the benthic foraminifer Cibicidoides wuellerstorfi are presented for the first time and interpreted along with Na/Ca and Sr/Ca as proxies of salinity and continental input, respectively. Our results highlight several warm (93, Gi15-19, and N1) and cool (92, M2, Gi20, and CN4) marine isotope stages (MISs). Accordingly, changes in the circulation of deep-water masses during the CAS closure developed in four main time intervals: (I) between 5.2 and 4.1 Ma (million years ago) BWT was ~1.1°C, (II) 4.1–3.2 Ma ~2.1°C, (III) 3.2–2.7 Ma ~2.7°C, and (IV) 2.7–2.2 Ma ~2.1°C. Relatively higher, gradually increased temperatures between 3.2 and 2.7 Ma correspond to late Pliocene warmth and restricted inflow of Pacific waters into the Caribbean due to shoaling of the CAS. In addition, Sr/Ca values reveal gradually escalating terrigenous input until 2.7 Ma most likely related to the increased river discharge in response to the Andean uplift. The gradual decrease of the BWTs from 2.7 Ma may have resulted from the onset of Northern Hemisphere Glaciation. Overall, BWTs match with previous sea surface temperatures from the planktic foraminifer Neogloboquadrina dutertrei. The BWTs presented here confirm intensified thermohaline circulation during the overall Pliocene warmth with increased bottom-water Na/Ca values indicating enhanced salinity. ©2020. The Authors.
Midlatitude Temperature Variations in the Oligocene to Early Miocene
Guitián, José; Phelps, Samuel; Polissar, Pratigya J.; et al. (2019)
Paleoceanography and Paleoclimatology
Decreasing Atmospheric CO2 During the Late Miocene Cooling
Tanner, Thomas; Hernández-Almeida, Iván; Drury, Anna J.; et al. (2020)
Paleoceanography and Paleoclimatology
A pronounced late Miocene cooling (LMC) from ~7 to 5.7 Ma has been documented in extratropical and tropical sea surface temperature records, but to date, available proxy evidence has not revealed a significant pCO2 decline over this event. Here, we provide a new, high-resolution pCO2 proxy record over the LMC based on alkenone carbon isotopic fractionation (εp) measured in sediments from the South Atlantic at Ocean Drilling Program (ODP) Site 1088. We apply a recent proxy calibration derived from a compilation of laboratory cultures, which more accurately reflects the proxy sensitivity to pCO2 changes during late Quaternary glacial-interglacial cycles, together with new micropaleontological proxies to reconstruct past variations in algal growth rate, an important secondary influence on the εp. Our resulting pCO2 record suggests an approximately twofold to threefold decline over the LMC and confirms a strong coupling between climate and pCO2 through the late Miocene. Within this long-term trend are pCO2 variations on sub-myr timescales that may reflect 400-kyr long-eccentricity cycles, in which pCO2 minima coincide with several orbital-scale maxima in published high-resolution benthic δ18O records. These may correspond to ephemeral glaciations, potentially in the Northern Hemisphere. Our temperature and planktonic δ18O records from Site 1088 are consistent with substantial equatorward movement of Southern Ocean frontal systems during the LMC. This suggests that potential feedbacks between cooling, ocean circulation and deep ocean CO2 storage may warrant further investigation during the LMC. ©2020. American Geophysical Union. All Rights Reserved.
A Saltier Glacial Mediterranean Outflow
Van Dijk, Joep; Ziegler, Martin; De Nooijer, Lennart J.; et al. (2018)
Paleoceanography and Paleoclimatology
High‐Resolution C‐Isotope, TOC and Biostratigraphic Records of OAE 1a (Aptian) From an Expanded Hemipelagic Cored Succession, Western Tethys: A New Stratigraphic Reference for Global Correlation and Paleoenvironmental Reconstruction
Castro, José M.; Ruiz-Ortiz, Pedro A.; de Gea, Ginés A.; et al. (2021)
Paleoceanography and Paleoclimatology
A high‐resolution carbonate C‐isotope stratigraphy for the Aptian is presented for the Cau core (Spain). The biostratigraphically calibrated C‐isotope stratigraphy of the core is used to refine the previously defined C‐isotope segments of the Aptian. Thirteen C‐isotope segments have been identified and correlated, and further subdivisions are presented. Correlation with other sections worldwide demonstrates the robustness of the C‐isotope stratigraphy of the Cau core. The studied succession includes a continuous record of the early Aptian Oceanic Anoxic Event (OAE 1a). Its onset has been studied at an ultrahigh‐resolution scale (0.2–0.5 kyr spacing), revealing a succession of sharp δ13Ccarb negative spikes, interpreted as a record of pulses of volcanism and methane emissions. The largest spike was rapid (<10 kyr) and marks the base of OAE 1a, which occurs within a longer‐term falling δ13Ccarb trend. The C‐isotope profile across OAE 1a perfectly records the negative (C3/Ap3), positive (C4/Ap4), steady (C5/Ap5), and positive (C6/Ap6) segments that were defined from Cismon (Italy) and subsequently identified worldwide. The Ap7 to Ap14 segments record a C‐isotope negative excursion, coupled with high TOC contents, probably related to regional paleogeography. The links with global environmental changes, episodes of widespread deposition of organic matter, and ultimately to major volcanic episodes are discussed. We propose the Cau core as a new reference section for the Aptian, and specifically for OAE 1a, based on its expanded and well‐preserved sedimentary, geochemical and biotic archives, which provide further insights into the environmental and biotic changes that occurred during this time interval. © 2021 American Geophysical Union

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