Sediment microhabitat effects on carbon stable isotopic signatures of microcosm‐cultured benthic foraminifera

Abstract

Environmentally controlled sedimentary microcosms were used to experimentally determine temperature, salinity, and depth-horizon effects on 613C DIC profiles of dissolved inorganic carbon (DIC) in sediment pore water (PW) and near-bottom water (BW). Simultaneously, benthic foraminifera1 populations of Ammonia beccarii were cultured and assayed for calcite 613C to establish how accurately foraminifera1 shells recorded BW-PW 813CDIC. One treatment population was restricted to the O-l -cm sediment depth; in another treatment, the population could freely "roam" up and down in the sediment microcosm. Pore-water DIC between the uppermost O-0.5-cm sediment layer and the overlying BW (i.e. 3 cm above the sediment-water interface) showed steep gradients of core-top AS13Cpw averaging -3.6-11.6Ym at 25°C and -2.6+0.5% at 20°C. Correspondingly, shell calcite 613C for A. beccarii collected from sediment microhabitats recorded a strong PW influence; V3C shell values averaged 1.1+0.8% lighter than BW V3C,, but 1.6 -L 0.6% enriched relative to average O-5-mm PW 613CDIC. Foraminifera restricted to the uppermost l-cm sediment depth throughout their lives exhibited shell calcite AV3C values not significantly different from foraminifera allowed to freeroam through the sediment column.Since the advent of stable isotopic investigation of calcitic microfossils in the 195Os, great effort has been directed toward exploiting stable isotopic information to reconstruct past oceanic and climatic conditions (e.g. Epstein et al. 1953; Emiliani 195 5;Duplessey et al. 1970). Such predictions are possible because the carbon and oxygen isotopic compositions of biogenic carbonates often exhibit fairly consistent relationships to the properties of ambient waters at the time of precipitation (Emiliani 195 5;O'Neil et al. 1969;Romanek et al. 1992). The application of these relationships to paleoceanic reconstructions via microfossils is based on the assumption that biogenic carbonate is precipitated either in equilibrium with the isotopic ratios of the ambient seawater environment or in consistent disequilibrium.When these assumptions are Acknowledgments

Tolerance of benthic foraminifera (Protista:Sarcodina) to hydrogen sulphide

“…However, a! !hough fcrs=inifera were observed not to r~p r o d u c e in the absence of sediment, they did so in the presence of a thin film of sediment and phytoplankton food ( < l mm thick; Chandler et al 1996, this study). r h a n 5~5 found in the foraminiferal assemblaqe under oxic conditions were evidently inhibited by sulphide in the sulphidic treatment and prolonged exposure to sulphidic conditions was most probably fatal to all foraminifera.…”

A physicochemically constrained seawater culturing system for production of benthic foraminifera

“…Our foraminiferal stock culture system contains natural sediment cultures in a recirculating, filtered artificial seawater system (Chandler and Green 1996); adult and juvenile Bulimina spp., Uvigerina peregrina, Cibicidoides spp., Melonis pompiloides, Bolivina spp., Discorbinella spp., and Hoeglundina elegans are typically found in these cultures, indicating that all these species are able to reproduce and grow in laboratory environments. When adults of three species were maintained in silica monocultures, Bulimina spp.…”

“…Each size fraction was transferred to a Tupperware ® container (~25 × 12 × 3 cm) and maintained at 5°C for the remainder of the cruise and during transport to the laboratory. After returning to the laboratory, these sievedsediment cultures were maintained following methods described by Chandler and Green (1996) and Chandler et al (1996). These stock cultures provided foraminifers for experiments using the trace-metal-controlled recirculating culture system.…”

“…However, the inability to maintain viable benthic foraminiferal cultures in physicochemically stable conditions has limited the success of culturing experiments on benthic species. Three studies have provided preliminary validation of stable isotope and trace metal proxies in benthic foraminifers, but these studies also demonstrated the need for improved culturing methods (Chandler et al 1996;Wilson-Finelli et al 1998;Havach et al 2001). These culturing studies used natural sediment substrates that contributed to microhabitat effects and diagenetic artifacts.…”

Modelling δ13C in benthic foraminifera: Insights from model sensitivity experiments

“…Hannah et al (1994); Nomaki et al (2007); Geslin et al (2011);Glas et al (2012)). Since it is notoriously difficult to culture deep-sea benthic foraminifera in the laboratory under insitu conditions, culture experiments are often limited to shallowwater species (Chandler et al, 1996), or specimen taken from water depths shallower than 250 m (Wilson-Finelli et al, 1998;Havach et al, 2001). Culturing systems like those developed by Hintz et al (2004) have allowed for systematic experiments on deep sea benthic foraminifera (Nomaki et al, 2005(Nomaki et al, , 2006McCorkle et al, 2008;Barras et al, 2010;Filipsson et al, 2010).…”