HR: 09:00h
AN: PP31A-05    [PDF]
TI: Testing Methods for Reconstruction of Degree of Calcite Saturation/Changes in Carbonate Ion Concentration in the Deep Sea Over the Last 25000 Years
AU: * Mekik, F A
EM: mekikf@gvsu.edu
AF: Grand Valley State University, Department of Geology, Allendale, MI 49401 United States
AU: Loubere, P W
EM: paul@geol.niu.edu
AF: Northern Illinois University, Department of Geology and Environmental Geosciences, De Kalb, IL 60115 United States
AU: Archer, D E
EM: d-archer@uchicago.edu
AF: University of Chicago, Department of Geophysical Sciences, Chicago, IL 60637 United States
AB: Reconstruction of changes in deep-sea calcite saturation is key to testing proposed mechanisms for changing atmospheric CO2 content on the glacial/interglacial timescale. The reconstruction has been hampered by difficulty in finding adequate quantitative proxies for calcite dissolution at the seabed. We test the sensitivity of two methods in estimating % calcite dissolved at the sea floor, and we examine the application of these methods to reconstruction of calcite saturation/carbonate ion concentration from the LGM to Present in the Eastern Equatorial Pacific [EEP]. The methods we examined are the G. menardii fragmentation index of Mekik et al. [2002] and the foraminiferal weight index of Broecker and Clark [2001]. The first method is calibrated to estimate % calcite dissolved at the seabed, the second method is calibrated to estimate downcore changes in carbonate ion concentration. Combination of the first method with reverse modeling using the Muds geochemical model and estimates of paleo-organic carbon flux allows downcore estimates of paleo-delta calcite. We examined the response of the proxies to modern calcite saturation using Archer's gridded geochemical data and found that foraminiferal weights for most species so far tested have little relationship to delta calcite. However, N. dutertrei shows a consistent pattern with most weight loss early in the dissolution process [when % calcite dissolved is less than 40%] and relatively little specimen weight change as delta calcite drops below zero. The menardii index, by contrast, shows strong linear sensitivity in the negative delta calcite range. These results indicate that the specimen weight method is most sensitive at higher delta calcite values; while the menardii index is most responsive in undersaturated conditions. Application of the methods downcore in the EEP reveals a consistent LGM to Present pattern in two cores for the menardii-reverse modeling approach to estimating paleo-delta calcite. The dutertrei-weight method yields opposite results for the two cores even though they must sample the same deep Pacific water mass. The discrepant results support work which indicates that specimen weight is influenced by foraminifera habitat upper ocean ecology [sub-species] and geochemistry [pCO2].
DE: 3030 Micropaleontology
DE: 4231 Equatorial oceanography
DE: 4267 Paleoceanography
DE: 4806 Carbon cycling
DE: 4842 Modeling
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting