Reconstruction of upwelling and productivity in the southern part of the Peru-Chile Current: A multi parameter approach

Five sediment cores were investigated with the aim to reconstruct the present and past regional upwelling and productivity variations in the southern part of the Peru-Chile Current (PCC). Several parameters such as organic carbon, calcium carbonate, opal, isotopic and faunal composition of planktic foraminifera, and the siliceous plankton assemblage served as proxies for the production and sedimentation of organic matter. The observations reveal generally higher productivities during the last glacial compared to the Early and Middle Holocene. During the Late Holocene, productivity increased again regardless of the position of the investigated cores. In addition, the observed predominantly southward increase in paleoproductivity, as recorded under present-day conditions, points to the same driving mechanisms of productivity in the last 40,000 years. During the last glacial, however, significant discrepancies can be observed between the record at 33°S and other records further north. Highest productivities at 33°S occurred around the Last Glacial Maximum (LGM, 21,000 ± 1000 yr BP), whereas further north, the productivity was highest before and after the LGM, and decreased slightly during the LGM.The applied productivity proxies showed that next to latitudinal position and strength of the zonal systems, i.e. the Southern Westerlies and the Antarctic Circumpolar Current (ACC), other processes such as the strength of the South Pacific subtropical gyre, El Niño Southern Oscillation (ENSO) events, and the hemispheric thermal gradient have been strongly affecting the upwelling intensity and paleoproductivity in this region during the last 40,000 years. In addition, other episodic events such as flooding of the shelf during Termination I (18,000 10,000 yr BP), as well as humid phases onshore prior to the LGM and during Termination I, may have increased the nutrient supply into the upwelling area north of 33°S resulting in enhanced paleoproductivity

Intensification of the East Australian Current After ∼1400 CE

The East Australian Current (EAC) is the western boundary current of the South Pacific Subtropical Gyre that transports warm tropical waters to higher southern latitudes and significantly impacts the climate of Australia and New Zealand. Modern observations show that the EAC has strengthened with rising global temperatures. However, little is known about the pre-industrial variability of the EAC and the forcing mechanisms. Planktic foraminifera Globigerinoides ruber (white) Mg/Ca-based sea surface temperature reconstructions offshore northeastern Australia between 15° and 26°S reveal an increase by ∼1.2°C after ∼1400 CE. We infer that the increase in temperature is related to a stronger EAC heat transport that is likely driven by a strengthening of the Southern Hemisphere subtropical gyre circulation due to a progressive shift of the Southern annular mode toward its positive phase and of El Niño-Southern Oscillation toward more El Niño-like conditions

Late Holocene slowdown of the Indian Ocean Walker circulation

Changes in tropical zonal atmospheric (Walker) circulation induce shifts in rainfall patterns along with devastating floods and severe droughts that dramatically impact the lives of millions of people. Historical records and observations of the Walker circulation over the 20th century disagree on the sign of change and therefore, longer climate records are necessary to better project tropical circulation changes in response to global warming. Here we examine proxies for thermocline depth and rainfall in the eastern tropical Indian Ocean during the globally colder Last Glacial Maximum (19–23 thousand years ago) and for the past 3000 years. We show that increased thermocline depth and rainfall indicate a stronger-than-today Walker circulation during the Last Glacial Maximum, which is supported by an ensemble of climate simulations. Our findings underscore the sensitivity of tropical circulation to temperature change and provide evidence for a further weakening of the Walker circulation in response to greenhouse warming

Deglacial δ18O and hydrologic variability in the tropical Pacific and Indian Oceans

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 387 (2014): 240–251, doi:10.1016/j.epsl.2013.11.032.Evidence from geologic archives suggests that there were large changes in the tropical hydrologic cycle associated with the two prominent northern hemisphere deglacial cooling events, Heinrich Stadial 1 (HS1; ∼19 to 15 kyr BP; kyr BP = 1000 yr before present) and the Younger Dryas (∼12.9 to 11.7 kyr BP). These hydrologic shifts have been alternatively attributed to high and low latitude origin. Here, we present a new record of hydrologic variability based on planktic foraminifera-derived δ18O of seawater (δ18Osw) estimates from a sediment core from the tropical Eastern Indian Ocean, and using 12 additional δ18Osw records, construct a single record of the dominant mode of tropical Eastern Equatorial Pacific and Indo-Pacific Warm Pool (IPWP) hydrologic variability. We show that deglacial hydrologic shifts parallel variations in the reconstructed interhemispheric temperature gradient, suggesting a strong response to variations in the Atlantic Meridional Overturning Circulation and the attendant heat redistribution. A transient model simulation of the last deglaciation suggests that hydrologic changes, including a southward shift in the Intertropical Convergence Zone (ITCZ) which likely occurred during these northern hemisphere cold events, coupled with oceanic advection and mixing, resulted in increased salinity in the Indonesian region of the IPWP and the eastern tropical Pacific, which is recorded by the δ18Osw proxy. Based on our observations and modeling results we suggest the interhemispheric temperature gradient directly controls the tropical hydrologic cycle on these time scales, which in turn mediates poleward atmospheric heat transport.ThisworkwasfundedbytheNationalScienceFoundation;theOceanandClimateChangeInstituteandtheAcademicProgramsOfficeatWoodsHoleOceano-graphicInstitution;BMBF(PABESIA);andDFG(He3412/15-1

Climatic controls on leaf wax hydrogen isotope ratios in terrestrial and marine sediments along a hyperarid-to-humid gradient

The hydrogen isotope composition of leaf wax biomarkers (δ2Hwax) is a valuable tool for reconstructing continental paleohydrology, since it serves as a proxy for the hydrogen isotope composition of precipitation (δ2Hpre). To yield robust palaeohydrological reconstructions using δ2Hwax in marine archives, it is necessary to examine the impacts of regional climate on δ2Hwax and assess the similarity between marine sedimentary δ2Hwax and the source of continental δ2Hwax. Here, we examined an aridity gradient from hyperarid to humid along the Chilean coast. We sampled sediments at the outlets of rivers draining into the Pacific as well as soils within catchments and marine surface sediments adjacent to the outlets of the studied rivers and analyzed the relationship between climatic variables and δ2Hwax values. We found that apparent fractionation between leaf waxes and source water is relatively constant in humid and semiarid regions (average: −121 ‰). However, it becomes less negative in hyperarid regions (average: −86 ‰) as a result of evapotranspirative processes affecting soil and leaf water 2H enrichment. We also observed that along strong aridity gradients, the 2H enrichment of δ2Hwax follows a non-linear relationship with water content and water flux variables, driven by strong soil evaporation and plant transpiration. Furthermore, our results indicate that δ2Hwax values in marine surface sediments largely reflect δ2Hwax values from the continent, confirming the robustness of marine δ2Hwax records for paleohydrological reconstructions along the Chilean margin. These findings also highlight the importance of considering the effects of hyperaridity in the interpretation of δ2Hwax values and pave the way for more quantitative paleohydrological reconstructions using δ2Hwax

Wetland expansion on the continental shelf of the northern South China Sea during deglacial sea level rise

To identify environmental causes for past changes in vegetation in subtropical East Asia, we present carbon isotope compositions of plant-wax n-alkanes and provide estimates of the C4-plant contribution across the past four glacial terminations and interglacials, based on cores recovered from the northern South China Sea. Our results show a comparable C4-plant contribution between the Last Glacial Maximum (LGM) and the Holocene. An increase of the C4-plant contribution by 15–20% is found for Terminations IV, II and I relative to subsequent interglacial peaks, coeval with an expansion of Cyperaceae and Poaceae. In contrast, Termination V reveals a lower C4-plant contribution than Marine Isotope Stage (MIS) 11c. The data exhibit a long-term trend, with a stepwise increase of the C4-plant contribution across interglacials MIS 11c, 9e, 7e and 1. We suggest that no substantial changes in humidity levels over glacial-interglacial cycles occurred facilitating a similar C3/C4-plant ratio for the LGM and the Holocene. Instead, deglacial sea-level rises caused an extensive development of floodplains and wetlands on the exposed continental shelf, providing habitats for the spread of C4 sedges and grasses. The progressive subsidence of Chinese coastal areas and the broadening of the continental shelf over the late Quaternary explains the nearly absence of C4 plant occurrence during Termination V and a gradual increase of the C4-plant contribution across interglacial peaks. Taken together, changes in coastal environments should be considered when interpreting marine-based vegetation reconstructions from subtropical Asia

Deciphering the variability in Mg/Ca and stable oxygen isotopes of individual foraminifera

oraminifera are commonly used in paleoclimate reconstructions as they occur throughout the world's oceans and are often abundantly preserved in the sediments. Traditionally, foraminifera‐based proxies like δ18O and Mg/Ca are analyzed on pooled specimens of a single species. Analysis of single specimens of foraminifera allows reconstructing climate variability on timescales related to El Niño‐Southern Oscillation (ENSO) or seasonality. However, quantitative calibrations between the statistics of individual foraminiferal analyses (IFA) and climate variability are still missing. We performed Mg/Ca and δ18O measurements on single specimens from core‐top sediments from different settings to better understand the signal recorded by individual foraminifera. We used three species of planktic foraminifera (G. ruber (s.s.), T. sacculifer, and N. dutertrei) from the Indo‐Pacific Warm Pool (IPWP) and one species (G. ruber (pink)) from the Gulf of Mexico (GoM). Mean values for the different species of Mg/Ca vs calculated δ18O temperatures agree with published calibration equations. IFA statistics (both mean and standard deviation) of Mg/Ca and δ18O between the different sites show a strong relationship indicating that both proxies are influenced by a common factor, most likely temperature variations during calcification. This strongly supports the use of IFA to reconstruct climate variability. However, our combined IFA data for the different species only show a weak relationship to seasonal and interannual temperature changes, especially when seasonal variability increases at a location. This suggests that the season and depth habitat of the foraminifera strongly affect IFA variability, such that ecology needs to be considered when reconstructing past climate variability

North Atlantic forcing of tropical Indian Ocean climate

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 509 (2014): 76-80, doi:10.1038/nature13196.The response of the tropical climate in the Indian Ocean realm to abrupt climate change events in the North Atlantic Ocean is contentious. Repositioning of the intertropical convergence zone is thought to have been responsible for changes in tropical hydroclimate during North Atlantic cold spells1–5, but the dearth of high-resolution records outside the monsoon realm in the Indian Ocean precludes a full understanding of this remote relationship and its underlying mechanisms. Here we show that slowdowns of the Atlantic meridional overturning circulation during Heinrich stadials and the Younger Dryas stadial affected the tropical Indian Ocean hydroclimate through changes to the Hadley circulation including a southward shift in the rising branch (the intertropical convergence zone) and an overall weakening over the southern Indian Ocean. Our results are based on new, high-resolution sea surface temperature and seawater oxygen isotope records of well dated sedimentary archives from the tropical eastern Indian Ocean for the past 45,000 years, combined with climate model simulations of Atlantic circulation slowdown under Marine Isotope Stages 2 and 3 boundary conditions. Similar conditions in the east and west of the basin rule out a zonal dipole structure as the dominant forcing of the tropical Indian Ocean hydroclimate of millennial-scale events. Results from our simulations and proxy data suggest dry conditions in the northern Indian Ocean realm and wet and warm conditions in the southern realm during North Atlantic cold spells.This study was funded by the German Bundesministerium für Bildung und Forschung (grant 03G0189A) and the Deutsche Forschungsgemeinschaft (DFG grants HE3412/15-1 and STE1044/4-1, and the DFG Research Centre/Cluster of Excellence ‘The Ocean in the Earth System’). D.W.O. is funded by the US NSF, R.D.P.-H. is supported by Chilean FONDAP 15110009/ICM Nucleus NC120066.2014-10-3