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

Abstract

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.

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

“…The predictions were run with of 1.66‰ and 0.86‰ (Vienna Standard Mean Ocean Water), based on the observed mean and range for the site. These predictions used HadISST monthly average SSTs (Rayner et al, ), which reasonably replicated buoy‐measured SST seasonality (Richey et al, ). All errors are ±1 standard deviation.…”

“…Solid green bars are the site modern annual or seasonal SST means ± 1 standard deviation, using monthly (1981–2017) NOAA Optimum Interpolation Sea Surface Temperature V2 (Reynolds et al, ). (b) The annual hierarchical calibration used to predict for G. ruber in the northern Gulf of Mexico, compared with averaged sediment trap observations (2010–2013) from Richey et al (). The predictions were run with of 1.66‰ and 0.86‰ (Vienna Standard Mean Ocean Water), based on the observed mean and range for the site.…”

“…In this example we test how well our calibration can replicate the seasonality of monthly measured from G. ruber (white and pink) at a sediment trap site, where SSTs and values are well constrained (Figure b). The site is in the northern Gulf of Mexico and has repeated measurements for each month from 2010 to 2013 (Richey et al, ). These years are pooled into monthly mean values.…”

“…We use the hierarchical annual calibration and not the seasonal variant because the seasonal calibrations favor SSTs associated with high foraminifera abundance and so tend to be better at predicting peak abundance seasons rather than growth for each month of the year. We use HadISST (Rayner et al, ) monthly mean SSTs from the nearest grid point to the sediment trap site, as Richey et al () found that HadISST reasonably replicated the seasonality of SSTs when compared with measurements from a local buoy. We create two predictions each with values of 1.40‰ (VSMOW) and 0.86‰ (VSMOW), following the measured mean of 1.13‰ and annual range of 0.53‰ between 0 and 50m depth (Richey et al, ).…”

Reconstructing 800 Years of Carbonate Ion Concentration in the Cariaco Basin Using the Area Density of Planktonic Foraminifera Shells

“…Richey et al () recently showed that there are two morphotypes of G. ruber (pink), corresponding to the sensu stricto and sensu latu morphotypes of G. ruber (white). While previous studies have shown that calcification can vary significantly between morphotypes (Marshall et al, ; Osborne et al, ), the Marshall et al () relationship, generated prior to the recognition of G. ruber (pink) morphotypes, proved robust without distinguishing between the two.…”