Benthic foraminiferal shell weight: Deglacial species-specific responses from the Santa Barbara Basin

Abstract

Here we present a record of size-normalized shell weight for four species of benthic foraminifera through a period of rapid environmental change during the most recent deglaciation (Santa Barbara Basin, CA). A strong Oxygen Minimum Zone (OMZ), the product of high surface productivity and poor ventilation, characterizes the eastern Pacific; this subsurface zone is mechanistically coupled with high concentrations of dissolved inorganic carbon. The OMZ migrated vertically during warming of the last deglaciation, leading to rapid shifts in the oxygenation and inorganic carbon system of the benthos. The size-normalized weight (SNW) of benthic foraminifers Uvigerina peregrina, Bolivina interjuncta, and Bolivina tumida reflect only the broad trends of the vertical migration of the OMZ, and inorganic carbon system, overshadowed by clear species-specific trends. The relative importance of OMZ migrations versus other environmental variables and optimal growth conditions differs across species of benthic foraminifera. In U. peregrina, SNW primarily peaks with foraminiferal density and increased abundance of that species, while B. interjuncta and B. tumida increase in SNW with a shrinking of the OMZ (and carbon maximum) in the late Holocene. Bolivina argentea shows no long-term trends in SNW potentially due to its ability to migrate through the sediment. Our results suggest that, while inorganic carbon and dissolved oxygen may play a role in determining shell weight across species of benthic foraminifera, neither parameter alone is responsible for changes in benthic foraminiferal shell weight in the fossil record.

Enhanced Carbonate Dissolution Associated With Deglacial Dysoxic Events in the Subpolar North Pacific

“…Size and growth rate in marine benthic organisms can vary along oxygen gradients and will increase or decrease with oxygenation depending on the physiological preferences of the species (Belanger et al., 2020; Glock et al., 2019; Keating‐Bitonti & Payne, 2017). In U. peregrina from GoA, shell volumes are greatest where Mo/Al and the relative abundance of dysoxia‐tolerant foraminifera are highest (Figure 3), similar to results from SBB (Davis et al., 2016), which indicates an association between larger U. peregrina shells and lower oxygen environments. Foraminifera capable of using nitrate as their electron acceptor in metabolic reactions may grow faster in low‐oxygen environments and Uvigerina spp.…”

“…However, SBW does not account for the size variation among individuals within the sieved size fraction. MBW typically normalizes weight using a one‐dimensional measurement of the specimens weighted within a narrow size fraction (Barker & Elderfield, 2002; Beer et al., 2010a; Todd et al., 2020), while others normalize weight to the relative area of two‐dimensional silhouettes of specimens among samples (Davis et al., 2016). The ρ A methods calculate density by dividing the weight of individuals by their silhouette areas and averaging individual densities within samples (Marshall et al., 2013).…”

“…In addition, other aspects of the growth habitat, including temperature, salinity, and food availability, may drive changes in foraminiferal growth and calcification, and thus influence SNW (Beer et al, 2010b;Bijma et al, 1990;Broecker & Clark, 2001;de Villiers, 2004;Pak et al, 2018;Todd et al, 2020;Weinkauf et al, 2016). In some paleoceanographic records, dissolution proxies such as fragmentation counts and external smoothing of shell features show a decoupling between dissolution and SNW (Davis et al, 2016;Pak et al, 2018;Todd et al, 2020 counts as an independent measure of dissolution and found that variation in SNW could not be explained by variation in dissolution intensity and instead interpreted SNW in terms of local growth conditions. In the Santa Barbara Basin (SBB) prior to 1975, G. bulloides SNWs were highest during times with upwelling of cool waters with lower Ω and showed no external signs of dissolution, suggesting that growth responses to temperature had a greater influence on SNW than carbonate chemistry (Pak et al, 2018).…”

“…In addition, other aspects of the growth habitat, including temperature, salinity, and food availability, may drive changes in foraminiferal growth and calcification, and thus influence SNW (Beer et al., 2010b; Bijma et al., 1990; Broecker & Clark, 2001; de Villiers, 2004; Pak et al., 2018; Todd et al., 2020; Weinkauf et al., 2016). In some paleoceanographic records, dissolution proxies such as fragmentation counts and external smoothing of shell features show a decoupling between dissolution and SNW (Davis et al., 2016; Pak et al., 2018; Todd et al., 2020). For example, Todd et al.…”

“…For example, growth in planktonic foraminifera with symbionts may be less affected by low‐pH conditions than in nonsymbiotic species (Lombard et al., 2010; Manno et al., 2012). Studies using benthic foraminifera have also found inconsistencies in SNW changes among species indicating that species have dissimilar responses, perhaps due to the different microhabitats the species prefer during calcification (Davis et al., 2016). Thus, initial calcification may reflect a multitude of environmental and ecological factors and, thus, SNW may covary with factors other than carbonate ion saturation state at the time of calcification or after death.…”