Reconstructing the thermal structure of the upper ocean : insights from planktic foraminifera shell chemistry and alkenones in modern sediments of the tropical eastern Indian Ocean

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 26 (2011): PA3219, doi:10.1029/2011PA002132.Shell chemistry of planktic foraminifera and the alkenone unsaturation index in 69 surface sediment samples in the tropical eastern Indian Ocean off West and South Indonesia were studied. Results were compared to modern hydrographic data in order to assess how modern environmental conditions are preserved in sedimentary record, and to determine the best possible proxies to reconstruct seasonality, thermal gradient and upper water column characteristics in this part of the world ocean. Our results imply that alkenone-derived temperatures record annual mean temperatures in the study area. However, this finding might be an artifact due to the temperature limitation of this proxy above 28°C. Combined study of shell stable oxygen isotope and Mg/Ca ratio of planktic foraminifera suggests that Globigerinoides ruber sensu stricto (s.s.), G. ruber sensu lato (s.l.), and G. sacculifer calcify within the mixed-layer between 20 m and 50 m, whereas Globigerina bulloides records mixed-layer conditions at ∼50 m depth during boreal summer. Mean calcifications of Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, and Globorotalia tumida occur at the top of the thermocline during boreal summer, at ∼75 m, 75–100 m, and 100 m, respectively. Shell Mg/Ca ratios of all species show a significant correlation with temperature at their apparent calcification depths and validate the application of previously published temperature calibrations, except for G. tumida that requires a regional Mg/Ca-temperature calibration (Mg/Ca = 0.41 exp (0.068*T)). We show that the difference in Mg/Ca-temperatures of the mixed-layer species and the thermocline species, particularly between G. ruber s.s. (or s.l.) and P. obliquiloculata, can be applied to track changes in the upper water column stratification. Our results provide critical tools for reconstructing past changes in the hydrography of the study area and their relation to monsoon, El Niño-Southern Oscillation, and the Indian Ocean Dipole Mode.This project was funded by the German Ministry of Education and Research (BMBF project PABESIA) and the “Deutsche Forschungsgemeinschaft” (DFG project HE 3412/15–1)

Glacial to Holocene swings of the Australian–Indonesian monsoon

Author Posting. © The Author(s), 2011. 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 Geoscience 4 (2011): 540–544, doi:10.1038/ngeo1209.The Australian-Indonesian monsoon is an important component of the climate system in the tropical Indo-Pacific region. However, its past variability, relation with northern and southern high latitude climate and connection to the other Asian monsoon systems are poorly understood. Here we present high-resolution records of monsoon-controlled austral winter upwelling during the past 22,000 years, based on planktic foraminiferal oxygen isotope and faunal composition in a sedimentary archive collected offshore southern Java. We show that glacial-interglacial variations in the Australian-Indonesian winter monsoon were in phase with the Indian summer monsoon system, consistent with their modern linkage through cross-equatorial surface winds. Likewise, millennial-scale variability of upwelling shares similar sign and timing with upwelling variability in the Arabian Sea. On the basis of element composition and grain-size distribution as precipitation-sensitive proxies in the same archive, we infer that (austral) summer monsoon rainfall was highest during the Bølling-Allerød period and the past 2,500 years. Our results indicate drier conditions during Heinrich Stadial 1 due to a southward shift of summer rainfall and a relatively weak Hadley Cell south of the Equator. We suggest that the Australian-Indonesian summer and winter monsoon variability were closely linked to summer insolation and abrupt climate changes in the northern hemisphere.This study was funded by the German Bundesministerium für Bildung und Forschung (PABESIA) and the Deutsche Forschungsgemeinschaft (DFG, HE 3412/15-1). DWO’s participation was funded by the U.S. National Science Foundation

El Niño variability off Peru during the last 20,000 years

Here we present a high-resolution marine sediment record from the El Niño region off the coast of Peru spanning the last 20,000 years. Sea surface temperature, photosynthetic pigments, and a lithic proxy for El Niño flood events on the continent are used as paleo–El Niño–Southern Oscillation proxy data. The onset of stronger El Niño activity in Peru started around 17,000 calibrated years before the present, which is later than modeling experiments show but contemporaneous with the Heinrich event 1. Maximum El Niño activity occurred during the early and late Holocene, especially during the second and third millennium B.P. The recurrence period of very strong El Niño events is 60–80 years. El Niño events were weak before and during the beginning of the Younger Dryas, during the middle of the Holocene, and during medieval times. The strength of El Niño flood events during the last millennium has positive and negative relationships to global and Northern Hemisphere temperature reconstructions

Ablagerung und Frühdiagenese organischen Materials in marinen Hochproduktivitätsgebieten

This study incorporates a detailed interdisciplinary approach involving geochemistry and microscopy to characterise organic matter in upwelling sediments along the continental margins of Peru and Oman. The sediments were drilled during 'Ocean Drilling Program (ODP)'-Leg 112 (Sites 679, 681 and 688) and Leg 117 (Sites 720, 723 and 724), respectively. In addition sampIes from a high productivity area off Pakistan in the northem Arabian Sea were inc1uded to enable a comparison. The purpose of these investigations were to provide an insight into the origin of sedimentary organic matter in marine high productivity areas and subsequently to integrate these findings into sedimentation models and to demonstrate sedimentation processes. Over 250 samples from 14 different bore holes were investigated. The microscopic and organic/inorganic geochemical methods utilized were: Reflected light microscopy, transmitted light microscopy, transmission electron microscopy (TEM) , scanning electron microscopy (SEM), elemental analysis of organic carbon, sulfur, iron and nitrogen, Rock-Eval pyrolysis, pyrolysis-gas-chromatography and kinetic experiments of the hydrocarbon generation potential. The high content of organic carbon (up to 10 wt.-%) found in sediments deposited under an intense oxygen minimum zone suggested in the past that oxygen deficient environments are important for the preservation of organic matter. However, microscopic and nanoscopic investigations revealed that the structural integrity of primary organic matter is slight. Furthermore, it appears that in sediments under the immediate influence of high productivityareas, microbially catalyzed degradation processes within the water column result in the decay of primary organic material. The organic matter in sediments deposited undemeath such high productivity zones consist mainly (up to 99 vol.-%) of unstructured organic aggregates. In sediments outside these upwelling zones terrigenous organic particles are predominant. The primary marine organic matter has been almost completely remineralized. The fraction of organic matter reaching the sediment surface was calculated to be at the most 10 %, even if the bioproductivity in the photic layer of the water column is high. At most investigated locations, further oxidation reactions of organic matter in the sediment led to oxygen depletion, resulting in the establishment of an anoxie environment and the initiation of anaerobic degradation processes. This anoxic zone was found a few centimeters below the sediment/water-interface. The deposition of organic matter shows a broad spatial and temporal variability. The geochemical investigations suggest that the depositional conditions offshore Peru are less variable than those from the Arabian Sea off Oman. The studied sedimentary sequence off Oman can be subdivided into three intervals which are characterized by different environmental settings. In the oldest interval (upper Pliocene - lower Pleistocene) the sediments were deposited within a small marginal basin. Water circulation and erosion of sedimentary matter were restricted by a structural barrier. High productivity of surface waters resulted in the [...