Polissar Pratigya J.

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Polissar
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Pratigya J.
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  • Article
    Relationship between individual chamber and whole shell Mg/Ca ratios in Trilobatus sacculifer and implications for individual foraminifera palaeoceanographic reconstructions
    (Nature Research, 2021-01-11) Rustic, Gerald T. ; Polissar, Pratigya J. ; Ravelo, Ana Christina ; de Menocal, Peter B.
    Precisely targeted measurements of trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reveal inter-chamber heterogeneities in specimens of the planktic foraminifer Trilobatus (Globigerinoides) sacculifer. We find that Mg/Ca ratios in the final growth chamber are generally lower compared to previous growth chambers, but final chamber Mg/Ca is elevated in one of thirteen sample intervals. Differences in distributions of Mg/Ca values from separate growth chambers are observed, occurring most often at lower Mg/Ca values, suggesting that single-chamber measurements may not be reflective of the specimen’s integrated Mg/Ca. We compared LA-ICPMS Mg/Ca values to paired, same-individual Mg/Ca measured via inductively coupled plasma optical emission spectrometry (ICP-OES) to assess their correspondence. Paired LA-ICPMS and ICP-OES Mg/Ca show a maximum correlation coefficient of R = 0.92 (p < 0.05) achieved by applying a weighted average of the last and penultimate growth chambers. Population distributions of paired Mg/Ca values are identical under this weighting. These findings demonstrate that multi-chamber LA-ICPMS measurements can approximate entire specimen Mg/Ca, and is thus representative of the integrated conditions experienced during the specimen’s lifespan. This correspondence between LA-ICPMS and ICP-OES data links these methods and demonstrates that both generate Mg/Ca values suitable for individual foraminifera palaeoceanographic reconstructions.
  • Article
    Low-frequency orbital variations controlled climatic and environmental cycles, amplitudes, and trends in northeast Africa during the Plio-Pleistocene
    (Nature Research, 2023-10-09) Lupien, Rachel ; Uno, Kevin ; Rose, Cassaundra ; deRoberts, Nicole ; Hazan, Cole ; deMenocal, Peter B. ; Polissar, Pratigya J.
    The eastern Mediterranean sapropels, paced by insolation, provide a unique archive of African monsoon strength over the Late Neogene. However, the longer-term climate of this region lacks characterization within the context of changes in ice volume, sea surface temperature gradients, and terrestrial ecosystems. Here, we examine C28 n-alkanoic acid leaf wax hydrogen and carbon isotopes in sapropels, sourced from northeast Africa, along with vegetation-corrected precipitation isotopes, derived from astronomically dated sediment cores from ODP 160 Sites 966 and 967 since 4.5 million years ago. Despite sampling only wet-phase sapropels for African monsoon variability, we find a larger range in hydrogen isotopes than previously published data across wet-dry precession cycles, indicating the importance of long-term modulation of Green Sahara phases throughout the Neogene. An influence of orbital properties on regional monsoonal hydroclimate is observed, controlling up to 50% of total hydrogen isotope variance, but large changes outside of these typical frequencies account for at least 50% of the total variance. This secular trend may track changes in ice volume, tropical sea surface temperature, sea surface temperature gradients, or even lower-frequency orbital cycles. Long-term hydroclimate and environmental shifts provide new contexts for milestone events in northeast African hominin dispersal and evolution.