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Geochemical records generated from the calcite shells (tests) of benthic foraminifera, especially those of the genera Cibicidoides and Uvigerina, provide the basis of the majority of long-term climate records in a variety of proxy reconstructions. However, the extent to which benthic foraminifera are affected by post-depositional alteration is poorly constrained in the literature. Furthermore, how diagenesis may alter the geochemical composition of benthic foraminiferal tests, and thereby biasing a variety of proxy-based climate records, is also poorly constrained. We present the Foraminiferal Preservation Index (FPI) as a new metric to quantify preservation quality based on objective, well-defined criteria. The FPI is used to identify and quantify trends in diagenesis temporally, from modern coretop samples to the Mid-Pliocene Warm Period (0.0-3.3 million year ago), and spacially in the deep ocean. The FPI identifies the chemical composition of deep ocean water masses to be the primary driver of diagenesis through time, while also serving as a supplementary method of identifying periods of changing water mass influence at a given site through time. Additionally, we present stable isotope data (d18O, d13C) generated from individual Cibicidoides tests of various preservation quality that demonstrate the likelihood of significant biasing in a variety of geochemical proxy records, especially those used to reconstruct past changes in ice volume and sea level. These single-test data also demonstrate the robustness of paleorecords generated from carefully selected specimens of only the highest quality of preservation.
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Changes in benthic foraminifera assemblage preservation associated with onset of deep AMOC during the last deglaciation
Benthic foraminifera are used to generate the majority of paleo-proxy records reconstructing past ocean changes including variations in the strength of AMOC. To assess the reliability of geochemical proxy records generated using benthic foraminifera, a Foraminifera Preservation Index (FPI) was developed to quantify assemblage-wide changes in visual preservation quality. The qualitative criteria for preservation included in the FPI are supported by stable isotope and trace element datasets. Early application of the FPI on Cibicidoidesassemblages from the deep Pacific Ocean (IODP Sites 846, 1143, 1208) reveal quantifiably better preservation during glacial periods relative to interglacial periods for the last ~1 million years. Here, we present results from two summer REU projects tracking such preservation changes in the deep North and South Atlantic Ocean prior to and throughout the last deglaciation (~0-35 ka). Changes in Cibicidoides FPI from IODP Site 1089 in the deep South Atlantic (~4600m water depth: primarily bathed by Antarctic Bottom Water - AABW) mirror those in the Pacific with better preservation during the glacial maximum of Marine Isotope Stage (MIS 2) than the Holocene interglacial (MIS 1). Alternatively, Cibicidoides FPI from IODP Site 1059 (~3000m water depth: bathed by North Atlantic Deep Water [NADW] during interglacials; and by AABW during glacials) reveal better preservation during the Holocene relative to MIS 2. Despite these opposing trends, changes in FPI occur at both sites at ~15 ka corresponding to major changes to AMOC documented throughout the deep Atlantic basin. These findings imply that the same processes involved in water mass CO2-carbonate chemistry on glacial-interglacial timescales affect preservation of benthic foraminifera. Furthermore, our results suggest that the FPI can track major changes in deglacial AMOC, potentially providing an inexpensive method to produce preliminary data prior to or in unison with more expensive geochemical analyses.
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- Award ID(s):
- 1658230
- PAR ID:
- 10194357
- Date Published:
- Journal Name:
- AGU Fall Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Geochemical records generated from the calcite shells (tests) of benthic foraminifera, especially those of the genera Cibicidoides and Uvigerina, provide the basis of the majority of long-term climate records in a variety of proxy reconstructions. However, the extent to which benthic foraminifera are affected by post-depositional alteration is poorly constrained in the literature. Furthermore, how diagenesis may alter the geochemical composition of benthic foraminiferal tests, and thereby biasing a variety of proxy-based climate records, is also poorly constrained. We present the Foraminiferal Preservation Index (FPI) as a new metric to quantify preservation quality based on objective, well-defined criteria. The FPI is used to identify and quantify trends in diagenesis temporally, from modern coretop samples to the Mid-Pliocene Warm Period (0.0-3.3 million year ago), and spacially in the deep ocean. The FPI identifies the chemical composition of deep ocean water masses to be the primary driver of diagenesis through time, while also serving as a supplementary method of identifying periods of changing water mass influence at a given site through time. Additionally, we present stable isotope data (d18O, d13C) generated from individual Cibicidoides tests of various preservation quality that demonstrate the likelihood of significant biasing in a variety of geochemical proxy records, especially those used to reconstruct past changes in ice volume and sea level. These single-test data also demonstrate the robustness of paleorecords generated from carefully selected specimens of only the highest quality of preservation.more » « less
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Abstract Geochemical records generated from the calcite tests of benthic foraminifera, especially those of the generaCibicidoidesandUvigerina, provide the basis for proxy reconstructions of past climate. However, the extent to which benthic foraminifera are affected by postdepositional alteration is poorly constrained. Furthermore, how diagenesis may alter the geochemical composition of benthic foraminiferal tests, and thereby biasing a variety of proxy‐based climate records, is also poorly constrained. We present the Foraminiferal Preservation Index (FPI) as a new metric to quantify preservation quality based on objective, well‐defined criteria. The FPI is used to identify and quantify trends in diagenesis temporally, from late Pliocene to modern coretop samples (3.3–0 Ma), as well as spatially in the deep ocean. The FPI identifies the chemical composition of deep‐ocean water masses to be the primary driver of diagenesis through time, while also serving as a supplementary method of identifying periods of changing water mass influence at a given site. Additionally, we present stable isotope data (δ18O, δ13C) generated from individualCibicidoidesspecimens of various preservation quality that demonstrate the likelihood of significant biasing in a variety of geochemical proxy records, especially those used to reconstruct past changes in ice volume and sea level. These single‐test data further demonstrate that when incorporating carefully selected tests of only the highest preservation quality, robust paleorecords can be generated.more » « less
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The isotopic and chemical compositions of benthic foraminifera have been used for decades to deduce a broad variety of paleoclimate information. However, there has been little research as to what extent quantitative physical characteristics of benthic foraminifer shells such as their size and mass are related to the quality of their preservation. We used a large data set containing detailed information about individual shell weights, shell sizes, and preservational quality of fossil benthic foraminifera of the genus Cibicidoides from the Pacific ODP Site 846 and the Atlantic ODP Sites 929 and 1089, spanning the last deglaciation (~0-25 ka). We found that during both MIS 1 (~0-8 ka) and 2 (~18-25 ka), smaller and lighter Cibicidoides shells from Pacific Site 846 were typically better-preserved than shells from larger size fractions. Poorly-preserved shells from ODP Site 846 feature a higher mass/size ratio than their better-preserved counterparts that cannot be attributed to the filling of chambers by clay or other contaminant phases. Interestingly, opposite trends were observed at both Atlantic sites, where larger and heavier shells and shells exhibiting higher mass/size ratios are among the best-preserved. These findings point towards minute differences in the ontogenetic development of Cibicidoides shells from Atlantic and Pacific waters, allowing for a better presentation of certain size-ranges, and/or different mechanisms controlling preservation and diagenesis in Atlantic and Pacific deep waters. In addition to these mass/size metrics, we also examine the progression of diagenesis through internal wall structures via SEM images of shell cross sections, as well as the impact on trace metal concentrations measured via LA-ICPMS.more » « less
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