Abstract
AbstractStable oxygen and carbon isotope (δ18O and δ13C) values measured in foraminiferal calcite are one of the primary tools used in paleoceanography. Diagenetic recrystallization of foraminiferal calcite can act to reset primary isotopic values, but its effects are typically poorly quantified. Here we test the impact of early stage diagenesis on stable isotope records generated from a suite of drill sites in the equatorial Pacific Ocean recovered during Ocean Drilling Program Leg 199 and Integrated Ocean Drilling Program Expedition 320. Our selected sites form paleowater and burial depth transects, with excellent stratigraphic control allowing us to confidently correlate our records. We observe large intersite differences in the preservation state of benthic foraminiferal calcite, implying very different recrystallization histories, but negligible intersite offsets in benthic δ18O and δ13C values. We infer that diagenetic alteration of benthic foraminiferal calcite (in sedimentary oozes) must predominantly occur at shallow burial depths (<100 m) where offsets in both the temperature and isotopic composition of waters in which the foraminifera calcified and pore waters in which diagenesis occurs are small. Our results suggest that even extensive recrystallization of benthic foraminiferal calcite results in minimal shifts from primary δ18O and δ13C values. This finding supports the long‐held suspicion that diagenetic alteration of foraminiferal calcite is less problematic in benthic than in planktic foraminifera and that in deep‐sea sediments routinely employed for paleoceanographic studies benthic foraminifera are robust recorders of stable isotope values in the fossil record.
Highlights
[1] Stable oxygen and carbon isotope (δ18O and δ13C) values measured in foraminiferal calcite are one of the primary tools used in paleoceanography
Low Eocene benthic δ18O values measured at Ocean Drilling Program (ODP) Site 647 in the North Atlantic have been attributed to selective diagenetic overprinting of benthic and not planktic calcite but scanning electron microscope (SEM) analysis reveals little or no evidence of recrystallization in benthic and planktic foraminifer tests [Arthur et al, 1989; Pearson and Burgess, 2008]
Higher sedimentation rates will result in samples being more deeply buried, and recrystallization will occur at higher temperatures than found at the seafloor and diagenetic calcite will have a δ18O composition offset from biogenic calcite [Schrag et al, 1995; Sexton and Wilson, 2009]
Summary
[2] The stable isotopic composition of foraminiferal tests is a valuable archive for the reconstruction of paleoclimatic and paleoceanographic changes, and for global stratigraphic correlation. EDGAR ET AL.: DIAGENESIS IN BENTHIC FORAMINIFERA integrity is perhaps best illustrated by the “cool tropics paradox” [D’Hondt and Arthur, 1996; Wilson and Opdyke, 1996] This paradox describes the cool or similar-to-modern tropical sea surface temperature (SST) estimates derived from δ18O values of recrystallized or “frosty” planktic foraminiferal test calcite [Crowley and North, 1991; Price et al, 1998; Crowley and Zachos, 2000], in conflict with other lines of evidence for very warm Cretaceous and Paleogene tropics [Kolodny and Raab, 1988; Wilson and Opdyke, 1996; Andreasson and Schmitz, 1998]. The selected sites are stratigraphically continuous in the Oligocene and early Miocene (at least to magnetozone and biozone level), have benthic foraminifera present throughout, and most importantly, benefit from excellent stratigraphic control (uncertainty on magnetochron boundaries typically tens of centimeters)
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