Evaluating controls on planktonic foraminiferal geochemistry in the Eastern Tropical North Pacific

The oxygen isotopic composition of planktic foraminiferal calcite ( δ18Oc) is one of the most prevalent proxies used in the paleoceanographic community. The relationship between δ18Oc, temperature, and seawater oxygen isotopic composition ( δ18Ow) is firmly rooted in thermodynamics, and experimental constraints are commonly used for sea surface temperature (SST) reconstructions. However, in marine sedimentary applications, additional sources of uncertainty emerge, and these uncertainty constraints have not as of yet been included in global calibration models. Here, we compile a global data set of over 2,600 marine sediment core top samples for five planktic species: Globigerinoides ruber, Trilobatus sacculifer, Globigerina bulloides, Neogloboquadrina incompta, and Neogloboquadrina pachyderma. We developed a suite of Bayesian regression models to calibrate the relationship between δ18Oc and SST. Spanning SSTs from 0.0 to 29.5 °C, our annual model with species pooled together has a mean standard error of approximately 0.54‰. Accounting for seasonality and species‐specific differences improves model validation, reducing the mean standard error to 0.47‰. Example applications spanning the Late Quaternary show good agreement with independent alkenone‐based estimates. Our pooled calibration model may also be used for reconstruction in the deeper geological past, using modern planktic foraminifera as an analog for non‐extant species. Our core top‐based models provide a robust assessment of uncertainty in the δ18Oc paleothermometer that can be used in statistical assessments of interproxy and model‐proxy comparisons. The suite of models is publicly available as the Open Source software library bayfox, for Python, R, and MATLAB/Octave.

δ^{18} O_{c}

may be strongly skewed to record temperatures from cold, upwelled waters.…”

Section: Resultsmentioning

confidence: 99%

Global Core Top Calibration of δ¹⁸O in Planktic Foraminifera to Sea Surface Temperature

Malevich

Vetter

Tierney

2019

“…Both regressions indicate that Mg/Ca-temperature sensitivity that is much lower than the 9%/°C of the Anand et al (2003) equation, which is the most widely used for down core reconstruction of Mg/Ca-SST. Recent sediment trap studies have also found lower Mg/Ca-temperature sensitivity in G. ruber than the canonical 9%/°C, including a temperature sensitivity as low as 3%/°C in the Gulf of Tehuantepec (Gibson et al, 2016) and 6%/°C in a global sediment trap data set published by Gray et al (2018). These discrepancies likely stem from inherent uncertainties in estimating calcification temperature from δ 18 O or gridded temperature data sets, as well as secondary controls on foraminiferal Mg/Ca (e.g., salinity and pH).…”

Section: 1029/2018pa003417mentioning

confidence: 96%

Considerations for Globigerinoides ruber (White and Pink) Paleoceanography: Comprehensive Insights From a Long‐Running Sediment Trap

Richey

Thirumalai

Khider

et al. 2019

We present a detailed analysis of the seasonal distribution, size, morphological variability, and geochemistry of co‐occurring pink and white chromotypes of Globigerinoides ruber from a high‐resolution (1–2 weeks) and long‐running sediment trap time series in the northern Gulf of Mexico. We find no difference in the seasonal flux of the two chromotypes. Although flux of G. ruber is consistently lowest in winter, the flux‐weighted signal exported to marine sediments represents mean annual conditions in the surface mixed layer. We observe the same morphological diversity among pink specimens of G. ruber as white. Comparison of the oxygen and carbon isotopic composition (δ18O and δ13C) of two morphotypes (sensu stricto and sensu lato) of pink G. ruber reveals the isotopes to be indistinguishable. The test size distribution within the population varies seasonally, with the abundance of large individuals increasing (decreasing) with increasing (decreasing) sea surface temperature. We find no systematic offsets in the Mg/Ca and δ18O of co‐occurring pink and white G. ruber. The sediment trap data set shows that the Mg/Ca‐temperature sensitivity for both chromotypes is much lower than the canonical 9%/°C, which can likely be attributed to the secondary influence of both salinity and pH on foraminiferal Mg/Ca. Using paired Mg/Ca and δ18O, we evaluate the performance of a suite of published equations for calculating sea surface temperature, sea surface salinity, and isotopic composition of seawater (δ18Osw), including a new salinity‐δ18Osw relationship for the northern Gulf of Mexico from water column observations.

“…bulloides yielded the most robust validation statistics and suggested a temperature sensitivity of 7.9 ± 0.1% per degree Celsius that broadly agrees with previous calibrations, which show a relatively wide range of sensitivities. Culture (Mashiotta et al, ; Lea et al, ) and some sediment trap data (Jonkers et al, ; Pak et al, ) point to a higher T sensitivity of 9–10% per degree Celsius, while other sediment trap data (Gibson et al, ; McConnell & Thunell, ) and core‐top analyses of individual chambers (Marr et al, ) point to a lower sensitivity of ~6% per degree Celsius. All else being equal, calibrations across a wider T range should better resolve the true T sensitivity, and one possible explanation for the observed range in T sensitivity may be the T range over which calibrations were performed.…”

Section: Discussionmentioning

confidence: 99%

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

Saenger

Evans

2019

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 that Ω and 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.