Diagenesis of Lower Cretaceous pelagic carbonates, North Atlantic; paleoceanographic signals obscured

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
Pennsylvania State University, United States
U. S. Geological Survey, United States
Volume Title: Paleoecological and geochemical signatures of Cretaceous anoxic events; a tribute to William V. Sliter
Volume Author(s): Huber, Brian T., editor; Bralower, Timothy J.; Leckie, R. Mark
Source: Journal of Foraminiferal Research, 29(4), p.340-351; Geological Society of America, 30th annual meeting, Toronto, ON, Canada, Oct. 26-29, 1998, edited by Brian T. Huber, Timothy J. Bralower and R. Mark Leckie. Publisher: Cushman Foundation for Foraminiferal Research, Ithaca, NY, United States. ISSN: 0096-1191 CODEN: JFARAH
Note: In English. 51 refs.; illus., incl. 1 table
Summary: The stable isotope and minor element geochemistry of Neocomian (Lower Cretaceous) pelagic carbonates of the North Atlantic Basin (Deep Sea Drilling Project Sites 105, 367, 387, 391, and 603) was examined 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 of pre-Aptian age, in which individual microfossils are not available for analysis. Data indicate that in addition to depth of burial, rhythmic variations in primary carbonate content have strongly controlled diagenetic patterns and associated geochemical signatures in these Neocomian sequences. Samples become increasingly depleted in Sr and δ18O with increasing CaCO3 content. Within individual sedimentary sections, substantial decreases in Sr/Ca ratios and δ18O values are evident over a range of 4 to 98% CaCO3. However, even over a relatively narrow range of 50 to 98% CaCO3 a 2.5 per mil variation in δ18O values and a change of a factor of 1.7 in Sr/Ca ratios are observed. Carbon isotope compositions do not vary as extensively with CaCO3 content, but carbonate-rich intervals tend to be relatively depleted in 13C. Petrographic analysis reveals that these geochemical patterns are related to the transfer of CaCO3 from carbonate-poor intervals (calcareous shales and marlstones) to adjacent carbonate-rich intervals (limestones) during burial compaction and pressure solution. This process results in the addition of diagenetic cement to carbonate-rich intervals to produce a bulk composition that is relatively depleted in Sr and 18O and, at the same time, enables the retention of more-or-less primary carbonate that is relatively enriched in Sr and 18O in adjacent carbonate-poor intervals. Thus, although cyclic variations in CaCO3 content are primary in the Neocomian sequences examined, measured variations in Sr/Ca ratios and δ18O values are not and, as such, do not provide reliable proxies for past variations in climate, oceanographic conditions, or global ice volume.
Year of Publication: 1999
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; Calcium; Calcium carbonate; Carbonate rocks; Chemical composition; Cretaceous; DSDP Site 105; DSDP Site 367; DSDP Site 387; DSDP Site 391; DSDP Site 603; Deep Sea Drilling Project; Diagenesis; Geochemistry; IPOD; Isotope ratios; Isotopes; Leg 11; Leg 41; Leg 43; Leg 44; Leg 93; Leg 95; Limestone; Lithofacies; Lower Cretaceous; Mesozoic; Metals; Minor elements; Neocomian; North Atlantic; O-18/O-16; Oxygen; Paleo-oceanography; Pelagic environment; Sedimentary rocks; Sr/Ca; Stable isotopes; Strontium
Coordinates: N345343 N345343 W0691024 W0691024
N122913 N122913 W0200250 W0200250
N321912 N321912 W0674000 W0674000
N281340 N281340 W0753653 W0753653
N352939 N352940 W0700142 W0700143
N352939 N384432 W0700142 W0723633
N352900 N384930 W0700100 W0724700
Record ID: 2000025810
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