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1. High‐Resolution Mg/Ca and δ 18 O Patterns in Modern Neogloboquadrina pachyderma From the Fram Strait and Irminger Sea

   https://doi.org/10.1029/2020PA003969  

   Livsey, Caitlin M. ; Kozdon, Reinhard ; Bauch, Dorothea ; Brummer, Geert‐Jan A. ; Jonkers, Lukas ; Orland, Ian ; Hill, Tessa M. ; Spero, Howard J. ( September 2020 , Paleoceanography and Paleoclimatology)

   Neogloboquadrina pachydermais the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ¹⁸O in discrete calcite zones using laser ablation‐inductively coupled plasma‐mass spectrometry, electron probe microanalysis, and secondary ion mass spectrometry within modernN. pachydermashells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity, and δ¹⁸O_swin which the shells calcified to better understand the controls onN. pachydermageochemical heterogeneity. We present a relationship between Mg/Ca and temperature inN. pachydermalamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable secondary ion mass spectrometry δ¹⁸O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ¹⁸O of the crust and lamellar calcite ofN. pachydermafrom an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts inN. pachydermaδ¹⁸O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions.

    

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2. Link between light-triggered Mg-banding and chamber formation in the planktic foraminifera Neogloboquadrina dutertrei

   https://doi.org/10.1038/ncomms15441  

   Fehrenbacher, Jennifer S. ; Russell, Ann D. ; Davis, Catherine V. ; Gagnon, Alexander C. ; Spero, Howard J. ; Cliff, John B. ; Zhu, Zihua ; Martin, Pamela ( May 2017 , Nature Communications)
3. High‐Resolution Mg/Ca and δ18O Patterns in Modern Neogloboquadrina pachyderma From the Fram Strait and Irminger Sea

   Livsey, C.M. ; Kozdon, R. ; Bauch, D. ; Brummer, G-J A. ; Jonkers, L. ; Orland, I. ; Hill, T. M. ; Spero, H.S. ( January 2020 , Paleoceanography and paleoclimatology)

   Abstract Neogloboquadrina pachyderma is the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ18O in discrete calcite zones using laser ablation‐inductively coupled plasma‐mass spectrometry, electron probe microanalysis, and secondary ion mass spectrometry within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity, and δ18Osw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. We present a relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable secondary ion mass spectrometry δ18O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ18O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma δ18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions. 

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4. Investigating ENSO‐Related Temperature Variability in Equatorial Pacific Core‐Tops Using Mg/Ca in Individual Planktic Foraminifera

   https://doi.org/10.1029/2019PA003774  

   Rongstad, Brigitta L. ; Marchitto, Thomas M. ; Serrato Marks, Gabriela ; Koutavas, Athanasios ; Mekik, Figen ; Ravelo, Ana Christina ( January 2020 , Paleoceanography and Paleoclimatology)

   El Niño Southern Oscillation (ENSO) is the largest source of interannual climate variability on Earth today; however, future ENSO remains difficult to predict. Evaluation of paleo‐ENSO may help improve our basic understanding of the phenomenon and help resolve discrepancies among models tasked with simulating future climate. Individual foraminifera analysis allows continuous down‐core records of ENSO‐related temperature variability through the construction and comparison of paleotemperature distributions; however, there has been little focus on calibrating this technique to modern conditions. Here, we present data from individual measurements of Mg/Ca in two species of planktic foraminifera, surface dwellingGlobigerinoides ruberand thermocline dwellingNeogloboquadrina dutertrei, from nine core tops across the equatorial Pacific (n≈70 per core for each species). Population variance, kernel probability density functions, and quantile‐quantile analyses are used to evaluate the shape of each Mg/Ca‐temperature distribution and to compare them to modern conditions using monthly temperatures from the Simple Ocean Data Assimilation. We show that populations of individual Mg/Ca measurements in bothG. ruberandN. dutertreireflect site‐specific temperature distribution shapes and variances across the equatorial Pacific when accounting for regional differences in depth habitats. Individual measurements of both taxa capture zonal increases in population variance from the western equatorial Pacific to the central equatorial Pacific and a spatially heterogeneous eastern equatorial Pacific, consistent with modern conditions. Lastly, we show that populations of individual Mg/Ca measurements are able to recover meaningful differences in temperature variability between sites within the eastern equatorial Pacific, lending support to this tool's application for paleo‐ENSO reconstructions.

    

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5. Calibration and Validation of Environmental Controls on Planktic Foraminifera Mg/Ca Using Global Core‐Top Data

   https://doi.org/10.1029/2018PA003507  

   Saenger, Casey P. ; Evans, Michael N. ( August 2019 , Paleoceanography and Paleoclimatology)

   The Mg/Ca of planktic foraminifera is commonly assumed to be a univariate function of ocean temperature, but recent work suggests that nonthermal variables may have a secondary effect, thereby complicating an inverse approach for paleotemperature reconstructions. However, the significance of secondary variables has not been independently validated, and their inclusion may reflect statistical overfitting. Here we evaluate the significance of seven predictive variables on a global compilation (n= 1,124) of core‐top planktic foraminifera Mg/Ca spanning five species. An additive approach was used to construct models that included only variables with significant validation skill. Optimal models support the use of Mg/Ca as a paleothermometer but also find evidence for nonthermal variables: (a) Mg/Ca (N. pachy+incompta) = 0.186 ± 0.001 · exp(0.095 ± 0.002 · T) + 2.95E‐3 ± 5.4E‐6 · size; (b) Mg/Ca (G. ruber) = 0.685 ± 0.005 · exp(0.058 ± 0.001 · T) + 0.928 ± 0.02 · Omega(deep); (c); Mg/Ca (G. inflata) = 1.737 ± 0.007 · exp(0.065 ± 0.001 · T) · Omega(shallow)^‐0.734 ± 0.003; and (d) Mg/Ca (G. bulloides) = 0.623 ± 0.007 · exp(0.079 ± 0.001 · T) + 0.548 ± 0.007 · Omega(deep). Results highlight the importance of independent validation and suggest thatΩand size dependences may complicate univariate inversions for paleotemperature in some species. This is particularly true over timescales when secondary terms cannot be considered constant. Forward modeling is an attractive alternative, and improved Mg/Ca models may reduce bias in paleoceanographic data assimilation efforts. Salinity is absent from all relationships, suggesting that it may have a smaller influence than previously thought or may not vary sufficiently to be resolved at core‐top sites. The stepwise approach illustrated in this study is widely applicable, and validation exercises should be applied to the calibration of other paleoceanographic proxies.

    

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