Interpreting geochemical signals in pelagic carbonates; controls on diagenetic signatures

Author(s): Frank, Tracy D.; Arthur, Michael A.; Dean, Walter E.
Author Affiliation(s): Primary:
University of Queensland, Department of Earth Sciences, Brisbane, Queensl., Australia
Other:
Pennsylvania State University, United States
U. S. Geological Survey, United States
Volume Title: Geological Society of America, 1998 annual meeting
Source: Abstracts with Programs - Geological Society of America, 30(7), p.55; Geological Society of America, 1998 annual meeting, Toronto, ON, Canada, Oct. 26-29, 1998. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: The stable isotope and minor element geochemistry of Neocomian (Lower Cretaceous) carbonates of the North Atlantic Basin (DSDP Sites 105, 367, 387, 391, and 603) was studied to develop a diagenetic model for pelagic limestones. In particular, we hoped to test the fidelity of whole-rock geochemical records as paleoceanographic indicators for pelagic deposits in which individual microfossils are not available for analysis. In agreement with previous diagenetic studies, our data indicate that the two major controls on the geochemical composition of pelagic limestones are primary carbonate content and depth of burial. With increasing CaCO3 content and burial depth, limestones become increasingly depleted in Sr and 18O. Within individual sedimentary sections, substantial variations in Sr/Ca ratios and in delta-18O values are evident over a range of 10 to 98% CaCO3. However, even over a relatively narrow range of 50 to 98% CaCO3, we observe, on average, a 2.5 per mill variation in δ18O values and changes of a factor of 2 in carbonate Sr/Ca ratios. Carbon isotope compositions do not vary as extensively with carbonate content, but CaCO3-rich intervals tend to be relatively depleted in 13C. Without consideration of the strong diagenetic overprint on geochemical signatures as a function of more-or-less primary variation in carbonate content, one could be led to interpret significant geochemical variations in terms of paleoceanographic processes. For example, because the terrigenous flux to deep-sea basins increases during sea level lowstands and decreases during highstands, basin-wide fluctuations in the CaCO3 content of pelagic limestones would be expected to correlate with sea level. Even without any significant primary variation, therefore, substantial variations in Sr/Ca and δ18O values would be expected to result from burial diagenesis. As such, although an intrabasinal correlation of geochemical patterns might correlate with sea level variations, neither the relative nor the absolute magnitudes of Sr/Ca ratios and δ18O values would reflect primary ocean-climate variations. Specific examples of how geochemical variations in pelagic carbonates may have already led to misinterpretation of Cretaceous oceanography and climate will be presented.
Year of Publication: 1998
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
Key Words: 02 Geochemistry; 12 Stratigraphy, Historical Geology and Paleoecology; Alkaline earth metals; Atlantic Ocean; C-13/C-12; Calcium; Calcium carbonate; Carbon; Carbonate rocks; Cretaceous; DSDP Site 105; DSDP Site 367; DSDP Site 387; DSDP Site 391; DSDP Site 603; Deep Sea Drilling Project; Diagenesis; Fluctuations; Geochemistry; IPOD; Interpretation; Isotope ratios; Isotopes; Leg 11; Leg 41; Leg 43; Leg 44; Leg 93; Leg 95; Limestone; Lower Cretaceous; Marine environment; Mesozoic; Metals; Minor elements; Models; North Atlantic; O-18/O-16; Oxygen; Paleo-oceanography; Paleoclimatology; Pelagic environment; Sedimentary rocks; Stable isotopes; Strontium; Variations; Whole rock
Coordinates: N345343 N345343 W0691024 W0691024
N122913 N122913 W0200250 W0200250
N321912 N321912 W0674000 W0674000
N281340 N281340 W0753653 W0753653
N352939 N352940 W0700142 W0700143
Record ID: 1999027309
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