Kristina A. Dahl

A multiple proxy and model study of Cretaceous upper ocean temperatures and atmospheric CO2 concentrations

foraminiferal d 18 O and Mg/Ca suggests that the ratio of magnesium to calcium in the Turonian-Coniacian ocean may have been lower than in the Albian-Cenomanian ocean, perhaps coincident with an ocean 87 Sr/ 86 Sr minimum. The carbon isotopic compositions of distinct marine algal biomarkers were measured in the same sediment samples. The d 13 C values of phytane, combined with foraminiferal d 13 C and inferred temperatures, were used to estimate atmospheric carbon dioxide concentrations through this interval. Estimates of atmospheric CO2 concentrations range between 600 and 2400 ppmv. Within the uncertainty in the various proxies, there is only a weak overall correspondence between higher (lower) tropical temperatures and more (less) atmospheric CO2. The GENESIS climate model underpredicts tropical Atlantic temperatures inferred from ODP Leg 207 foraminiferal d 18 O and Mg/Ca when we specify approximate CO2 concentrations estimated from the biomarker isotopes in the same samples. Possible errors in the temperature and CO2 estimates and possible deficiencies in the model are discussed. The potential for and effects of substantially higher atmospheric methane during Cretaceous anoxic events, perhaps derived from high fluxes from the oxygen minimum zone, are considered in light of recent work that shows a quadratic relation between increased methane flux and atmospheric CH4 concentrations. With 50 ppm CH4, GENESIS sea surface temperatures approximate the minimum upper ocean temperatures inferred from proxy data when CO2 concentrations specified to the model are near those inferred using the phytane d 13 C proxy. However, atmospheric CO2 concentrations of 3500 ppm or more are still required in the model in order to reproduce inferred maximum temperatures.

Sea surface temperature pattern reconstructions in the Arabian Sea

from throughout the Arabian Sea for four distinct time intervals (0 ka, 8 ka, 15 ka, and 20 ka) with the aim of understanding the history of the Indian Monsoon and the climate of the Arabian Sea region. This was accomplished through the use of paired Mg/Ca and d 18 O measurements of the planktonic foraminifer Globigerinoides ruber. By analyzing basin-wide changes and changes in cross-basinal gradients, we assess both monsoonal and regional-scale climate changes. SST was colder than present for the majority of sites within all three paleotime slices. Furthermore, both the Indian Monsoon and the regional Arabian Sea mean climate have varied substantially over the past 20 kyr. The 20 ka and 15 ka time slices exhibit average negative temperature anomalies of 2.5� –3.5� C attributable, in part, to the influences of glacial atmospheric CO2 concentrations and large continental ice sheets. The elimination of the cross-basinal SST gradient during these two time slices likely reflects a decrease in summer monsoon and an increase in winter monsoon strength. Changes in d 18 Ow that are smaller than the d 18 O signal due to global ice volume reflect decreased evaporation and increased winter monsoon mixing. SSTs throughout the Arabian Sea were still cooler than present by an average of 1.4� Ci n the 8 ka time slice. These cool SSTs, along with lower d 18 Ow throughout the basin, are attributed to stronger than modern summer and winter monsoons and increased runoff and precipitation. The results of this study underscore the importance of taking a spatial approach to the reconstruction of processes such as monsoon upwelling.

Reconstructing the phytoplankton community of the Cariaco Basin during the Younger Dryas cold event using chlorin steryl esters

[1] A record of the downcore distribution of chlorin steryl esters (CSEs) through the Younger Dryas was produced from Cariaco Basin sediments in order to assess the potential use of CSEs as recorders of the structure of phytoplankton communities through time. Using an improved high-performance liquid chromatography method for the separation of CSEs, we find significant changes in the distribution of CSEs during the Younger Dryas in the Cariaco Basin. During the Younger Dryas, enhanced upwelling in the Cariaco Basin caused an increase in the diatom population and therefore an increase in the relative abundance of CSEs derived from diatoms. In contrast, the dinoflagellate population, and therefore CSEs derived from dinoflagellates, decreased in response to the climate change during the Younger Dryas. These community shifts agree well with the shifts observed in the present day on a seasonal basis that result from the north-south migration of the Intertropical Convergence Zone over the Cariaco Basin. We also identify changes in the abundance of several CSEs that seem to reflect rapid warming and cooling events. This study suggests that CSEs are useful proxies for reconstructing phytoplankton communities and paleoenvironments. INDEX TERMS: 1055 Geochemistry: Organic geochemistry; 4267 Oceanography: General: Paleoceanography; 4855 Oceanography: Biological and Chemical: Plankton; KEYWORDS: Younger Dryas, Cariaco Basin, chlorin steryl esters