Climate impact on marine plankton dynamics during interglacials
Zusammenfassung der Projektergebnisse
This project provides a detailed model study of the development of ocean productivity during the warm climate intervals of the Holocene and the Eemian. For this purpose a number of climate- and ocean biogeochemical model simulations have been carried out and analyzed. It was found that although climate remained relatively stable in both periods at the global scale, larger reorganizations in spatial in temporal (seasonal, interannual) variability have occurred. One major aspect was the investigation of changes in seasonal variability, which may have implications for the use of paleo proxies as indicators of past sea surface temperatures (SST). Here, the export (= flux of material across the 100m depth level in the model) of CaCO3 was taken as the model variable most closely representing the coccolithophorid E. huxleyi and its vertical settling, which is the organism from which the Uk'37 index to reconstruct SSTs is derived, routinely. When compared to annual mean SSTs (SSTann), the calculated maximum calcite export flux-SSTs (SSTcamax) show strongly different behavior. First, there is a clear offset the SSTann, and this offset differs over time. This indicates that indeed in many places the major flux of alkenones is associated with a temperature that does not correspond to the annual mean. Furthermore, due to reorganizations in marine biology, the month of maximum export flux may change, potentially even without changing climate. This, however, may result in an apparent SST change in proxy records in the absence of a climate signal. Although it could be found that the seasonal temperature signals are less important for proxy calibration, their consideration during interpretation of the signal is still important and probably even more complex than thought before. The second major research topic of the present study was the investigation of the development of oxygen minimum zones (OMZ) in the ocean, which may serve as benchmark for the assessment of observations made in the recent decades. Here it was found that in the Eastern Tropical South Pacific (ETSP), an area where some paleo proxy data for the Holocene are available, the model reproduces a slight decrease in oxygen, which is in agreement with proxy data. When quantifying the internal variability of the ETSP OMZ, a clearly different behavior of the extent of hypoxic (<80 µmol l^-1) and suboxic (O2<30 µmol l^-1) water masses becomes evident. While hypoxic water masses are rather stable (+/- 3%) over the entire course of the Holocene, the volume of suboxic water masses exhibits much higher variability (+/- 20 %). This result suggests, that the observation of an expansion of the Eastern Tropical Pacific OMZ as made by Stramma et al. (2008) is probably indeed showing an anthropogenic signal, that is unprecedented during the Holocene, while for the detection of significant changes in suboxic water masses a stronger signal would be required.
Projektbezogene Publikationen (Auswahl)
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(2013): Last interglacial temperature evolution - a model inter-comparison. Climate of the Past, 9 (2). pp. 605-619
Bakker, P., E. J. Stone, S. Charbit, M. Groger, U. Krebs-Kanzow, S. P. Ritz, V. Varma, V. Khon, D. J. Lunt, U. Mikolajewicz, M. Prange, H. Renssen, B. Schneider, and M. Schulz
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(2014): The spatial-temporal patterns of Asian summer monsoon precipitation in response to Holocene insolation change: a model-data synthesis. Quaternary Science Reviews, 85: 47-62
Jin, L., B. Schneider, W. Park, M. Latif, V. Khon and X. Zhang
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(2015): The evolution of sub-monsoon systems in the Afro-Asian monsoon region during the Holocene - comparison of different transient climate model simulations. Climate of the Past, 11,305-326, doi:10.5194/cp-11-305-2015
Dallmeyer, A., M. Claussen, N. Fischer, K. Haberkorn, S. Wagner, M. Pfeiffer, L. Jin, V. Khon, Y. Wang, and U. Herzschuh
