Abstract As living organisms, planktonic foraminifera are not passive tracers of the environment. Their test geochemistry—arguably the single most important resource for paleoceanographic ...research—reflects the combined signal of environmental, biological, and preservational processes. For most species, comparisons of test stable isotopic composition within and among taxa provide the primary means for disentangling the relative influences of these different processes. Here we test the foundations of our paleoceanographic interpretations with the first quantitative comparison of the determinants of carbon and oxygen isotopic variation across multiple ocean basins, studies, and species by re‐analyzing size‐specific data collated from the literature. We find clear evidence of species‐specific biological effects (i.e., vital effects), as the intercepts of size‐specific carbon and oxygen isotopic compositions differ significantly among species. Trends in body size and isotopic composition, particularly in dinoflagellate bearing taxa, suggest that much of the size‐dependent isotopic variation observed in death assemblages (i.e., core tops and sediments) relates to factors influencing the maximum size obtained by adults rather than ontogeny. The presence and type of photosymbiont hosted (dinoflagellate, chrysophyte, or none) were a major factor affecting species‐ and size‐specific δ 18 O values. In contrast, size‐related trends in δ 13 C values were driven by depth habitat (mixed layer, thermocline, subthermocline), symbiont ecology and whether the assemblage was alive or dead when sampled. On this broad geographic and oceanographic scale, ocean basin and biome had a significant effect on δ 18 O and δ 13 C values . Our analysis and its model‐averaged predictions provide a quantitative basis for interpreting size‐specific isotopic variation in 22 species of modern macroperforate planktonic foraminifera. We conclude by highlighting existing data gaps and outstanding questions of the relative influence of environmental, preservational, and biological processes on variation in the test geochemistry of planktonic foraminifera.
Key Points First global compilation and statistical synthesis of controls on 13 C and 18 O The 18 O primarily shows basin differences and 13 C primarily shows depth habitat The 13 C offsets between same‐sized living and post‐reproductive dead individuals
Stable oxygen and carbon isotope (delta super(18)O and delta super(13)C) 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 delta super(18)O and delta super(13)C values. We infer that diagenetic alteration of benthic foraminiferal calcite (in sedimentary oozes) must predominantly occur at shallow burial depths (<100m) 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 delta super(18)O and delta super(13)C 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. Key Points * Different benthic foraminiferal taphonomies yield similar isotopic values * Benthic foraminiferal recrystallisation in oozes is at shallow burial depths * Benthic foraminifera from oozes are robust recorders of stable isotopes
Abstract
Stable oxygen and carbon isotope (
δ
18
O and
δ
13
C) 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
δ
18
O and
δ
13
C 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
δ
18
O and
δ
13
C 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.
Key Points
Different benthic foraminiferal taphonomies yield similar isotopic values
Benthic foraminiferal recrystallisation in oozes is at shallow burial depths
Benthic foraminifera from oozes are robust recorders of stable isotopes
As living organisms, planktonic foraminifera are not passive tracers of the environment. Their test geochemistry-arguably the single most important resource for paleoceanographic research-reflects ...the combined signal of environmental, biological, and preservational processes. For most species, comparisons of test stable isotopic composition within and among taxa provide the primary means for disentangling the relative influences of these different processes. Here we test the foundations of our paleoceanographic interpretations with the first quantitative comparison of the determinants of carbon and oxygen isotopic variation across multiple ocean basins, studies, and species by re-analyzing size-specific data collated from the literature. We find clear evidence of species-specific biological effects (i.e., vital effects), as the intercepts of size-specific carbon and oxygen isotopic compositions differ significantly among species. Trends in body size and isotopic composition, particularly in dinoflagellate bearing taxa, suggest that much of the size-dependent isotopic variation observed in death assemblages (i.e., core tops and sediments) relates to factors influencing the maximum size obtained by adults rather than ontogeny. The presence and type of photosymbiont hosted (dinoflagellate, chrysophyte, or none) were a major factor affecting species- and size-specific delta super(18)O values. In contrast, size-related trends in delta super(13)C values were driven by depth habitat (mixed layer, thermocline, subthermocline), symbiont ecology and whether the assemblage was alive or dead when sampled. On this broad geographic and oceanographic scale, ocean basin and biome had a significant effect on delta super(18)O and delta super(13)C values . Our analysis and its model-averaged predictions provide a quantitative basis for interpreting size-specific isotopic variation in 22 species of modern macroperforate planktonic foraminifera. We conclude by highlighting existing data gaps and outstanding questions of the relative influence of environmental, preservational, and biological processes on variation in the test geochemistry of planktonic foraminifera. Key Points * First global compilation and statistical synthesis of controls on super(13)C and super(18)O * The super(18)O primarily shows basin differences and super(13)C primarily shows depth habitat * The super(13)C offsets between same-sized living and post-reproductive dead individuals
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