Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to rec... more Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to reconstruct past surface ocean conditions, yet the shells of many planktic foraminifers are an aggregate mixture of multiple carbonate phases with differing δ 18 O compositions. Here we demonstrate how secondary ion mass spectrometry can be used to measure intrashell δ 18 O heterogeneity by performing in situ analyses on micrometer-scale (3-10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer. Secondary ion mass spectrometry measurements on shells taken from Holocene-aged sediments at three sites in different ocean basins confirm that the δ 18 O of gametogenic (GAM) calcite added to shells during the terminal (reproductive) stage of the life cycle is 1.0-1.4‰ higher than that of pregametogenic calcite. Examination of shells in cross section reveals that many have suffered varying degrees of internal dissolution, which further skews whole-shell δ 18 O compositions toward higher values by preferentially removing low δ 18 O pregametogenic calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high δ 18 O GAM calcite should be considered when assessing T. sacculifer δ 18 O records generated via conventional isotope ratio mass spectrometry. Plain Language Summary Foraminifera are unicellular marine organisms that grow microscopic shells made of the mineral calcite. Foraminifer shells are commonly preserved in deep-sea sediments, and their whole-shell chemistries are analyzed to reconstruct the history of ocean climate change. However, these analyses can be problematic because some foraminifera add multiple calcite phases with differing chemistries during their life cycle. Here we use a novel analytical technique called secondary ion mass spectrometry to measure oxygen isotope ratios on minute spots within individual shells of the planktic foraminifer, Trilobatus sacculifer. Our results show that whole T. sacculifer shells are a mixture of two geochemically distinct calcite phases-one with low oxygen isotope ratios that formed near the warm sea surface and one with high oxygen isotope ratios that formed as the shells sink into colder water. Hence, addition of secondary calcite in deeper water elevates whole-shell oxygen isotope values and introduces inaccuracies into sea surface reconstructions derived from conventional measurements. Images of the shells in cross section reveal that this inaccuracy in whole-shell measurements is compounded by the preferential dissolution of the internal calcite. Our study illustrates potential issues with whole-shell oxygen isotope measurements and demonstrates how secondary ion mass spectrometry analysis can improve the fidelity of sea surface reconstructions. WYCECH ET AL.
D. Clay Kelly conceptualized the study. J. Wycech hand-picked translucent and opaque planktic for... more D. Clay Kelly conceptualized the study. J. Wycech hand-picked translucent and opaque planktic foraminifera, processed 14 C data. J. Wycech and S. Marcott completed modeling calculations. J. Wycech wrote the manuscript in collaboration with D. Clay Kelly and S. Marcott. MATERIALS AND METHODS Samples used in this study are from a piston core (CH15-PC9-00, PC9 herein) recovered from atop the Blake Ridge in the northwestern Atlantic (lat/long: 31°55.691'N, 75°43.774'W, water depth: 2,790 meters, Fig. DR1). In total, the stratigraphic record (1,094 cm-thick) extends from the Holocene back to marine isotope stage (MIS) 6, and consists of hemi-pelagic, green clay with translucent and opaque foraminiferal shells co-occurring throughout. Weight percent CaCO 3 (%CaCO 3) was measured every 10 cm at Woods Hole Oceanographic Institute (WHOI) using a UIC Coulometrics Coulometer with an acidification unit providing an analytical precision of ≤0.2% (Table DR1). The δ 18 O measurements of T. sacculifer (250-350 µm) were performed at WHOI using a Finnigan MAT252 mass spectrometer fitted with a Kiel device. Measurement of the standard NBS-19 provided δ 18 O precision of 0.07‰ (Ostermann and Curry, 2000). Bulk-carbonate samples were prepared for 14 C analysis by weighing ~4 mg of powdered bulk sediment from a given core depth and oven drying at 30°C overnight. Each core sample was disaggregated using a pH-buffered sodium hexametaphosphate solution, wet sieved, rinsed with distilled water, oven-dried (30°C) overnight, and then dry sieved (>150 µm) to concentrate foraminifera. The shells selected for geochemical analysis were directly analyzed for 14 C, and
Pliocene equatorial Pacific sea surface temperatures were warmer than modern everywhere, with l... more Pliocene equatorial Pacific sea surface temperatures were warmer than modern everywhere, with largest anomalies in the east A mean El Niño-like state, characterized by a reduced zonal sea surface temperature difference, prevailed in the Pliocene equatorial Pacific Pliocene Indian Summer Monsoon is estimated to have been ~20-40% weaker than modern
A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) condit... more A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δO) and Mg/Ca compositions of shells belonging to the planktic foraminifer Trilobatus sacculifer. However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells of T. sacculifer from Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δO of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca rat...
The oxygen isotope (δO) compositions of final chamber fragments of individual shells of the plank... more The oxygen isotope (δO) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δO offset (ΔOSIMS-GSMS) of −0.9 ± 0.1‰ (± 2 SE). The volume of material analyzed in 10-μm SIMS spots is ~10 times smaller than that analyzed by GSMS; hence, the extent to which these ΔOSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSM...
Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to rec... more Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to reconstruct past surface ocean conditions, yet the shells of many planktic foraminifers are an aggregate mixture of multiple carbonate phases with differing δ 18 O compositions. Here we demonstrate how secondary ion mass spectrometry can be used to measure intrashell δ 18 O heterogeneity by performing in situ analyses on micrometer-scale (3-10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer. Secondary ion mass spectrometry measurements on shells taken from Holocene-aged sediments at three sites in different ocean basins confirm that the δ 18 O of gametogenic (GAM) calcite added to shells during the terminal (reproductive) stage of the life cycle is 1.0-1.4‰ higher than that of pregametogenic calcite. Examination of shells in cross section reveals that many have suffered varying degrees of internal dissolution, which further skews whole-shell δ 18 O compositions toward higher values by preferentially removing low δ 18 O pregametogenic calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high δ 18 O GAM calcite should be considered when assessing T. sacculifer δ 18 O records generated via conventional isotope ratio mass spectrometry. Plain Language Summary Foraminifera are unicellular marine organisms that grow microscopic shells made of the mineral calcite. Foraminifer shells are commonly preserved in deep-sea sediments, and their whole-shell chemistries are analyzed to reconstruct the history of ocean climate change. However, these analyses can be problematic because some foraminifera add multiple calcite phases with differing chemistries during their life cycle. Here we use a novel analytical technique called secondary ion mass spectrometry to measure oxygen isotope ratios on minute spots within individual shells of the planktic foraminifer, Trilobatus sacculifer. Our results show that whole T. sacculifer shells are a mixture of two geochemically distinct calcite phases-one with low oxygen isotope ratios that formed near the warm sea surface and one with high oxygen isotope ratios that formed as the shells sink into colder water. Hence, addition of secondary calcite in deeper water elevates whole-shell oxygen isotope values and introduces inaccuracies into sea surface reconstructions derived from conventional measurements. Images of the shells in cross section reveal that this inaccuracy in whole-shell measurements is compounded by the preferential dissolution of the internal calcite. Our study illustrates potential issues with whole-shell oxygen isotope measurements and demonstrates how secondary ion mass spectrometry analysis can improve the fidelity of sea surface reconstructions.
A controversial aspect of the Pliocene climate system is a posited permanent sea surface temperat... more A controversial aspect of the Pliocene climate system is a posited permanent sea surface temperature (SST) distribution resembling that during El Niño events, which is largely inferred from sea surface temperatures reconstructed from several sites in the equatorial Pacific. We utilize a reduced-dimension methodology on a compilation of previously published multiproxy (Mg/Ca, U k′ 37 , TEX 86 , and foraminifer assemblages) Pliocene SST records from the equatorial Pacific to reconstruct spatial and temporal snapshots of SST anomalies and a time series of Niño indices from 5 to 1 Ma. The use of multiple proxies increases the number of study sites and thereby improves the robustness of the reconstruction. We find that the early Pliocene equatorial Pacific was characterized by a reduced zonal SST difference due to minimal change in the west and extreme warmth in the east which peaked at 4.3 Ma. The intensity of this mean El Niño-like SST state then gradually diminished toward modern conditions. We also use the Pliocene Niño 4 time series to estimate the past strength of Indian Summer Monsoon given the modern correlation of it to the Niño 4 index. Results indicate the monsoon was weaker throughout the study interval with weakest conditions (~37% less rainfall than modern) occurring at 4.3 Ma, congruent with regional proxy records. In summation, this reduced-dimension approach spatially and temporally resolves the warm mean state of the Pliocene equatorial Pacific and has numerous applications to inferences of paleoclimate conditions in distal regions teleconnected to El Niño today. Plain Language Summary The Pliocene Epoch (5.3-2.6 million years ago) is the most recent time interval in Earth history when atmospheric carbon dioxide concentrations may have been similar to today and the continents were in their current configuration. For these reasons, Pliocene paleoclimate reconstructions are considered to be a useful indicator of conditions expected by the end of the 21st century. Several marine-derived Pliocene reconstructions from the equatorial Pacific suggest a sea surface temperature (SST) distribution that resembles SSTs during El Niño events today. El Niño events have widespread impacts including reduced marine productivity in the eastern equatorial Pacific and weakened Indian Summer Monsoon. Insights into Pliocene El Niño-like SSTs are typically based on paleoclimate reconstructions from a few sites that are then used to infer regional conditions. In this study, we apply a statistical method to a compilation of nine Pliocene SST records across the equatorial Pacific to fill in the spatial gaps in the paleotemperature reconstructions. Our maps of reconstructed Pliocene SSTs reveal that the eastern equatorial Pacific was 3-6°C warmer than today, which is consistent with a mean El Niño-like state. Given the modern El Niño-Indian Summer Monsoon relationship, we estimate that Pliocene monsoon was~20-40% weaker than today.
The oxygen isotope (δ18O) compositions of final chamber fragments of individual shells of the pla... more The oxygen isotope (δ18O) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δ 18 O offset (Δ18OSIMS-GSMS) of −0.9 ± 0.1‰ (± 2 SE). The volume of material analyzed in 10-μm SIMS spots is ~10^5 times smaller than that analyzed by GSMS; hence, the extent to which these Δ18OSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSMS δ18O difference include sample-standard mismatch by SIMS, differences in standardization of SIMS and GSMS, and non-calcite contaminants in samples. Although the two datasets are consistently offset, SIMS values reproduce inter-shell δ18O variability delineated by shell fragment GSMS values. This strong positive covariance proved useful for bringing the two datasets into agreement (i.e. Δ18OSIMS-GSMS = 0), and confirms that SIMS-based foraminifer δ18O values record changes in calcification temperature and/or δ18O of seawater. Whether shells of foraminifer taxa with differing microcrystalline structures, chemical composition, and/or preservation histories register a similar Δ18OSIMS-GSMS value is a subject of ongoing testing.
Radiocarbon (14 C) ages obtained from planktic foraminiferal calcite are a mainstay for reconstru... more Radiocarbon (14 C) ages obtained from planktic foraminiferal calcite are a mainstay for reconstructing ocean-climate change and carbon cycle dynamics of the past 30 k.y., yet the effects of diagenesis on this vital chronometer are poorly constrained. Here, we address this shortcoming by comparing 14 C ages and trace element ratios (Mg/Ca, Mn/Ca) of planktic foraminifera with white, opaque shells deemed well preserved by traditional standards to those with exquisitely preserved translucent shells. Results support a diagenetic mechanism as opaque shells yield 14 C ages invariably older and trace element ratios consistently higher than those of translucent shells. Radiocarbon age offsets are particularly pronounced in mono-specific samples taken from stratigraphic horizons proximal to the δ 18 O maximum marking the Last Glacial Maximum (LGM) and the subsequent deglacial. Radiocarbon-based calendar ages of translucent shells from the two intervals are congruent with the established age ranges for these climate events, whereas those of co-occurring opaque shells overestimate the LGM and deglacial by 8–15 k.y. and 14–22 k.y., respectively. These results demonstrate that the use of translucent foraminifera enhances reproducibility and accuracy of 14 C ages by minimizing the deleterious effects of diagenesis. This study serves as a cautionary tale since white, opaque foraminifera are common in pelagic sediments, and 14 C ages derived from their ostensibly well-preserved shells can lead to discrepancies in the timing of Quaternary climate events and ocean circulation reconstructions.
Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to rec... more Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to reconstruct past surface ocean conditions, yet the shells of many planktic foraminifers are an aggregate mixture of multiple carbonate phases with differing δ 18 O compositions. Here we demonstrate how secondary ion mass spectrometry can be used to measure intrashell δ 18 O heterogeneity by performing in situ analyses on micrometer-scale (3-10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer. Secondary ion mass spectrometry measurements on shells taken from Holocene-aged sediments at three sites in different ocean basins confirm that the δ 18 O of gametogenic (GAM) calcite added to shells during the terminal (reproductive) stage of the life cycle is 1.0-1.4‰ higher than that of pregametogenic calcite. Examination of shells in cross section reveals that many have suffered varying degrees of internal dissolution, which further skews whole-shell δ 18 O compositions toward higher values by preferentially removing low δ 18 O pregametogenic calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high δ 18 O GAM calcite should be considered when assessing T. sacculifer δ 18 O records generated via conventional isotope ratio mass spectrometry. Plain Language Summary Foraminifera are unicellular marine organisms that grow microscopic shells made of the mineral calcite. Foraminifer shells are commonly preserved in deep-sea sediments, and their whole-shell chemistries are analyzed to reconstruct the history of ocean climate change. However, these analyses can be problematic because some foraminifera add multiple calcite phases with differing chemistries during their life cycle. Here we use a novel analytical technique called secondary ion mass spectrometry to measure oxygen isotope ratios on minute spots within individual shells of the planktic foraminifer, Trilobatus sacculifer. Our results show that whole T. sacculifer shells are a mixture of two geochemically distinct calcite phases-one with low oxygen isotope ratios that formed near the warm sea surface and one with high oxygen isotope ratios that formed as the shells sink into colder water. Hence, addition of secondary calcite in deeper water elevates whole-shell oxygen isotope values and introduces inaccuracies into sea surface reconstructions derived from conventional measurements. Images of the shells in cross section reveal that this inaccuracy in whole-shell measurements is compounded by the preferential dissolution of the internal calcite. Our study illustrates potential issues with whole-shell oxygen isotope measurements and demonstrates how secondary ion mass spectrometry analysis can improve the fidelity of sea surface reconstructions. WYCECH ET AL.
D. Clay Kelly conceptualized the study. J. Wycech hand-picked translucent and opaque planktic for... more D. Clay Kelly conceptualized the study. J. Wycech hand-picked translucent and opaque planktic foraminifera, processed 14 C data. J. Wycech and S. Marcott completed modeling calculations. J. Wycech wrote the manuscript in collaboration with D. Clay Kelly and S. Marcott. MATERIALS AND METHODS Samples used in this study are from a piston core (CH15-PC9-00, PC9 herein) recovered from atop the Blake Ridge in the northwestern Atlantic (lat/long: 31°55.691'N, 75°43.774'W, water depth: 2,790 meters, Fig. DR1). In total, the stratigraphic record (1,094 cm-thick) extends from the Holocene back to marine isotope stage (MIS) 6, and consists of hemi-pelagic, green clay with translucent and opaque foraminiferal shells co-occurring throughout. Weight percent CaCO 3 (%CaCO 3) was measured every 10 cm at Woods Hole Oceanographic Institute (WHOI) using a UIC Coulometrics Coulometer with an acidification unit providing an analytical precision of ≤0.2% (Table DR1). The δ 18 O measurements of T. sacculifer (250-350 µm) were performed at WHOI using a Finnigan MAT252 mass spectrometer fitted with a Kiel device. Measurement of the standard NBS-19 provided δ 18 O precision of 0.07‰ (Ostermann and Curry, 2000). Bulk-carbonate samples were prepared for 14 C analysis by weighing ~4 mg of powdered bulk sediment from a given core depth and oven drying at 30°C overnight. Each core sample was disaggregated using a pH-buffered sodium hexametaphosphate solution, wet sieved, rinsed with distilled water, oven-dried (30°C) overnight, and then dry sieved (>150 µm) to concentrate foraminifera. The shells selected for geochemical analysis were directly analyzed for 14 C, and
Pliocene equatorial Pacific sea surface temperatures were warmer than modern everywhere, with l... more Pliocene equatorial Pacific sea surface temperatures were warmer than modern everywhere, with largest anomalies in the east A mean El Niño-like state, characterized by a reduced zonal sea surface temperature difference, prevailed in the Pliocene equatorial Pacific Pliocene Indian Summer Monsoon is estimated to have been ~20-40% weaker than modern
A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) condit... more A controversial aspect of Pliocene (5.3–2.6 Ma) climate is whether El Niño‐like (El Padre) conditions, characterized by a reduced trans‐equatorial sea‐surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea‐surface conditions derived from the oxygen isotope (δO) and Mg/Ca compositions of shells belonging to the planktic foraminifer Trilobatus sacculifer. However, fossil shells of this species are a mixture of multiple carbonate phases—pre‐gametogenic, gametogenic (reproductive), and diagenetic calcites—that formed under different physiological and/or environmental conditions and are averaged in conventional whole‐shell analyses. Through in situ measurements of micrometer‐scale domains within Pliocene‐aged shells of T. sacculifer from Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the δO of gametogenic calcite is 0.6–0.8‰ higher than pre‐gametogenic calcite, while the Mg/Ca rat...
The oxygen isotope (δO) compositions of final chamber fragments of individual shells of the plank... more The oxygen isotope (δO) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δO offset (ΔOSIMS-GSMS) of −0.9 ± 0.1‰ (± 2 SE). The volume of material analyzed in 10-μm SIMS spots is ~10 times smaller than that analyzed by GSMS; hence, the extent to which these ΔOSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSM...
Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to rec... more Oxygen isotope ratios (δ 18 O) measured from planktic foraminifer shells are commonly used to reconstruct past surface ocean conditions, yet the shells of many planktic foraminifers are an aggregate mixture of multiple carbonate phases with differing δ 18 O compositions. Here we demonstrate how secondary ion mass spectrometry can be used to measure intrashell δ 18 O heterogeneity by performing in situ analyses on micrometer-scale (3-10 μm) domains within individual shells of the extant, mixed layer species Trilobatus sacculifer. Secondary ion mass spectrometry measurements on shells taken from Holocene-aged sediments at three sites in different ocean basins confirm that the δ 18 O of gametogenic (GAM) calcite added to shells during the terminal (reproductive) stage of the life cycle is 1.0-1.4‰ higher than that of pregametogenic calcite. Examination of shells in cross section reveals that many have suffered varying degrees of internal dissolution, which further skews whole-shell δ 18 O compositions toward higher values by preferentially removing low δ 18 O pregametogenic calcite. The results of this study echo the calls of earlier studies cautioning that spatiotemporal changes in the proportion of high δ 18 O GAM calcite should be considered when assessing T. sacculifer δ 18 O records generated via conventional isotope ratio mass spectrometry. Plain Language Summary Foraminifera are unicellular marine organisms that grow microscopic shells made of the mineral calcite. Foraminifer shells are commonly preserved in deep-sea sediments, and their whole-shell chemistries are analyzed to reconstruct the history of ocean climate change. However, these analyses can be problematic because some foraminifera add multiple calcite phases with differing chemistries during their life cycle. Here we use a novel analytical technique called secondary ion mass spectrometry to measure oxygen isotope ratios on minute spots within individual shells of the planktic foraminifer, Trilobatus sacculifer. Our results show that whole T. sacculifer shells are a mixture of two geochemically distinct calcite phases-one with low oxygen isotope ratios that formed near the warm sea surface and one with high oxygen isotope ratios that formed as the shells sink into colder water. Hence, addition of secondary calcite in deeper water elevates whole-shell oxygen isotope values and introduces inaccuracies into sea surface reconstructions derived from conventional measurements. Images of the shells in cross section reveal that this inaccuracy in whole-shell measurements is compounded by the preferential dissolution of the internal calcite. Our study illustrates potential issues with whole-shell oxygen isotope measurements and demonstrates how secondary ion mass spectrometry analysis can improve the fidelity of sea surface reconstructions.
A controversial aspect of the Pliocene climate system is a posited permanent sea surface temperat... more A controversial aspect of the Pliocene climate system is a posited permanent sea surface temperature (SST) distribution resembling that during El Niño events, which is largely inferred from sea surface temperatures reconstructed from several sites in the equatorial Pacific. We utilize a reduced-dimension methodology on a compilation of previously published multiproxy (Mg/Ca, U k′ 37 , TEX 86 , and foraminifer assemblages) Pliocene SST records from the equatorial Pacific to reconstruct spatial and temporal snapshots of SST anomalies and a time series of Niño indices from 5 to 1 Ma. The use of multiple proxies increases the number of study sites and thereby improves the robustness of the reconstruction. We find that the early Pliocene equatorial Pacific was characterized by a reduced zonal SST difference due to minimal change in the west and extreme warmth in the east which peaked at 4.3 Ma. The intensity of this mean El Niño-like SST state then gradually diminished toward modern conditions. We also use the Pliocene Niño 4 time series to estimate the past strength of Indian Summer Monsoon given the modern correlation of it to the Niño 4 index. Results indicate the monsoon was weaker throughout the study interval with weakest conditions (~37% less rainfall than modern) occurring at 4.3 Ma, congruent with regional proxy records. In summation, this reduced-dimension approach spatially and temporally resolves the warm mean state of the Pliocene equatorial Pacific and has numerous applications to inferences of paleoclimate conditions in distal regions teleconnected to El Niño today. Plain Language Summary The Pliocene Epoch (5.3-2.6 million years ago) is the most recent time interval in Earth history when atmospheric carbon dioxide concentrations may have been similar to today and the continents were in their current configuration. For these reasons, Pliocene paleoclimate reconstructions are considered to be a useful indicator of conditions expected by the end of the 21st century. Several marine-derived Pliocene reconstructions from the equatorial Pacific suggest a sea surface temperature (SST) distribution that resembles SSTs during El Niño events today. El Niño events have widespread impacts including reduced marine productivity in the eastern equatorial Pacific and weakened Indian Summer Monsoon. Insights into Pliocene El Niño-like SSTs are typically based on paleoclimate reconstructions from a few sites that are then used to infer regional conditions. In this study, we apply a statistical method to a compilation of nine Pliocene SST records across the equatorial Pacific to fill in the spatial gaps in the paleotemperature reconstructions. Our maps of reconstructed Pliocene SSTs reveal that the eastern equatorial Pacific was 3-6°C warmer than today, which is consistent with a mean El Niño-like state. Given the modern El Niño-Indian Summer Monsoon relationship, we estimate that Pliocene monsoon was~20-40% weaker than today.
The oxygen isotope (δ18O) compositions of final chamber fragments of individual shells of the pla... more The oxygen isotope (δ18O) compositions of final chamber fragments of individual shells of the planktic foraminifer Orbulina universa were measured in situ via secondary ion mass spectrometry (SIMS) and by traditional gas-source mass spectrometry (GSMS) entailing acid digestion of sampled calcite. The paired SIMS-GSMS analyses were performed on final chamber fragments of fossil shells taken from the top of a sediment core (Holocene) as well as shells grown in laboratory culture. Multiple iterations of SIMS-GSMS analyses were conducted on final chamber fragments treated with a variety of cleaning protocols. The series of paired analyses yielded an average SIMS-GSMS δ 18 O offset (Δ18OSIMS-GSMS) of −0.9 ± 0.1‰ (± 2 SE). The volume of material analyzed in 10-μm SIMS spots is ~10^5 times smaller than that analyzed by GSMS; hence, the extent to which these Δ18OSIMS-GSMS values represent real differences in analyte vs. instrumental factors remains unclear. Possible contributing factors to the SIMS-GSMS δ18O difference include sample-standard mismatch by SIMS, differences in standardization of SIMS and GSMS, and non-calcite contaminants in samples. Although the two datasets are consistently offset, SIMS values reproduce inter-shell δ18O variability delineated by shell fragment GSMS values. This strong positive covariance proved useful for bringing the two datasets into agreement (i.e. Δ18OSIMS-GSMS = 0), and confirms that SIMS-based foraminifer δ18O values record changes in calcification temperature and/or δ18O of seawater. Whether shells of foraminifer taxa with differing microcrystalline structures, chemical composition, and/or preservation histories register a similar Δ18OSIMS-GSMS value is a subject of ongoing testing.
Radiocarbon (14 C) ages obtained from planktic foraminiferal calcite are a mainstay for reconstru... more Radiocarbon (14 C) ages obtained from planktic foraminiferal calcite are a mainstay for reconstructing ocean-climate change and carbon cycle dynamics of the past 30 k.y., yet the effects of diagenesis on this vital chronometer are poorly constrained. Here, we address this shortcoming by comparing 14 C ages and trace element ratios (Mg/Ca, Mn/Ca) of planktic foraminifera with white, opaque shells deemed well preserved by traditional standards to those with exquisitely preserved translucent shells. Results support a diagenetic mechanism as opaque shells yield 14 C ages invariably older and trace element ratios consistently higher than those of translucent shells. Radiocarbon age offsets are particularly pronounced in mono-specific samples taken from stratigraphic horizons proximal to the δ 18 O maximum marking the Last Glacial Maximum (LGM) and the subsequent deglacial. Radiocarbon-based calendar ages of translucent shells from the two intervals are congruent with the established age ranges for these climate events, whereas those of co-occurring opaque shells overestimate the LGM and deglacial by 8–15 k.y. and 14–22 k.y., respectively. These results demonstrate that the use of translucent foraminifera enhances reproducibility and accuracy of 14 C ages by minimizing the deleterious effects of diagenesis. This study serves as a cautionary tale since white, opaque foraminifera are common in pelagic sediments, and 14 C ages derived from their ostensibly well-preserved shells can lead to discrepancies in the timing of Quaternary climate events and ocean circulation reconstructions.
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Papers by Jody Wycech