A time series study of the carbon isotopic composition of deep-sea benthic foraminifera

[1] Variation of the delta(13)C of living (Rose Bengal stained) deep-sea benthic foraminifera is documented from two deep-water sites (similar to2430 and similar to3010 m) from a northwest Atlantic Ocean study area 275 km south of Nantucket Island. The carbon isotopic data of Hoeglundina elegans and Uvigerina peregrina from five sets of Multicorer and Soutar Box Core samples taken over a 10-month interval (March, May, July, and October 1996 and January 1997) are compared with an 11.5 month time series of organic carbon flux to assess the effect of organic carbon flux on the carbon isotopic composition of dominant taxa. Carbon isotopic data of Hoeglundina elegans at 3010 m show 0.3parts per thousand lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom. This delta(13)C change following the spring bloom is suggested to be due to the presence of a phytodetritus layer on the seafloor and the subsequent depletion of delta(13)C in the pore waters within the phytodetritus and overlying the sediment surface. Carbon isotopic data of H. elegans from the 2430 m site show an opposite pattern to that found at 3010 m with a delta(13)C enrichment following the spring bloom. This different pattern may be due to spatial variation in phytodetritus deposition and resuspension or to a limited number of specimens recovered from the March 1996 cruise. The delta(13)C of Uvigerina peregrina at 2430 m shows variation over the 10 month interval, but an analysis of variance shows that the variability is more consistent with core and subcore variability than with seasonal changes. The isotopic analyses are grouped into 100 mum size classes on the basis of length measurements of individual specimens to evaluate delta(13)C ontogenetic changes of each species. The data show no consistent patterns between size classes in the delta(13)C of either H. elegans or U. peregrina. These results suggest that variation in organic carbon flux does not preferentially affect particular size classes, nor do delta(13)C ontogenetic changes exist within the >250 to >750 mum size range for these species at this locality. On the basis of the lack of ontogenetic changes a range of sizes of specimens from a sample can be used to reconstruct delta(13)C in paleoceanographic studies. The prediction standard deviation, which is composed of cruise, core, subcore, and residual (replicate) variability, provides an estimate of the magnitude of variability in fossil delta(13)C data; it is 0.27parts per thousand for H. elegans at 3010 m and 0.4parts per thousand for U. peregrina at the 2430 m site. Since these standard deviations are based on living specimens, they should be regarded as minimum estimates of variability for fossil data based on single specimen analyses. Most paleoceanographic reconstructions are based on the analysis of multiple specimens, and as a result, the standard error would be expected to be reduced for any particular sample. The reduced standard error resulting from the analysis of multiple specimens would result in the seasonal and spatial variability observed in this study having little impact on carbon isotopic records.