Abstract
Applying the clumped isotope (Δ47) thermometer to foraminifer microfossils offers the potential to significantly improve paleoclimate reconstructions, owing to its insensitivity to the isotopic composition of seawater (unlike traditional oxygen isotope (δ18O) analyses). However, the extent to which primary Δ47 signatures of foraminiferal calcites can be altered during diagenesis is not well known. Here, we present Δ47 data as well as high-resolution (∼10 kyr) δ18O and δ13C middle Eocene time series, measured on benthic and planktic foraminifera from ODP/IODP Sites 1408, 1409, 1410, 1050, 1260 and 1263 in the Atlantic Ocean. The sites examined span various oceanographic regimes, including the western tropical to mid-latitude North Atlantic, and the eastern mid-latitude South Atlantic. Comparing data from contemporaneous foraminifera with different preservation states, we test the effects of diagenetic alteration on paleotemperature reconstructions for the deep and surface ocean. We find that overall, primary Δ47 signatures appear similarly sensitive to diagenetic overprinting as δ18O, with differences in sensitivity depending on pore fluid chemistry and the amount of secondary calcite. Where planktic foraminifera are significantly altered, sea surface temperatures derived from Δ47 and δ18O values are biased towards cool temperatures. In comparison, Δ47 and δ18O values of benthic and well preserved planktic foraminifera are less affected by diagenesis and thus likely to yield robust foraminiferal calcification temperatures. With independent estimates of diagenetic calcite fractions, secondary overprints could be corrected for, using end-member modeling and Δ47-based temperatures from benthic foraminifera.
Highlights
IntroductionNon-recrystallized pristine tests without structural alteration or overgrowths tend to have a translucent (‘‘glassy”) appearance under the binocular microscope, whereas recrystallized tests appear opaque (‘‘frosty”) (Sexton et al, 2006b)
Benthic foraminifera from Site 1409 appear exceptionally well preserved, whereas benthic foraminifera from Sites 1260 and 1263 are characterized by noticeably poorer states of preservation
This impression is confirmed by Scanning Electron Microscope (SEM) analysis, which reveals much smoother surface textures of benthic foraminiferal tests at Site 1409 in comparison to Sites 1260 and 1263 (Fig. 2)
Summary
Non-recrystallized pristine tests without structural alteration or overgrowths tend to have a translucent (‘‘glassy”) appearance under the binocular microscope, whereas recrystallized tests appear opaque (‘‘frosty”) (Sexton et al, 2006b). Surface microstructures such as pores and spines can be preserved even in the case of significant recrystallization. This makes it difficult to precisely assess neomorphic changes under the light microscope. Cementation occurs when carbonate mineral overgrowths form on or within tests during burial. Like recrystallization, this process impacts the bulk chemistry of foraminiferal tests. A series of studies have shown that the geochemical composition of fossils may be altered without visually perceptible changes, for example through solid state diffusion of isotopes at high burial temperatures (Passey and Henkes, 2012; Henkes et al, 2014; Shenton et al, 2015)
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