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

Transcription

1 PALEOCEANOGRAPHY, VOL. 17, NO. 3, 1036, /2001PA000664, 2002 A time series study of the carbon isotopic composition of deep-sea benthic foraminifera Bruce H. Corliss Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, USA Daniel C. McCorkle Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA David M. Higdon Institute of Statistics and Decision Sciences, Duke University, Durham, North Carolina, USA Received 8 June 2001; revised 13 March 2002; accepted 13 March 2002; published 14 August [1] Variation of the d 13 C of living (Rose Bengal stained) deep-sea benthic foraminifera is documented from two deep-water sites (2430 and 3010 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 0.3% lower mean values following an organic carbon flux maximum resulting from a spring phytoplankton bloom. This d 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 d 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 d 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 d 13 CofUvigerina 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 mm size classes on the basis of length measurements of individual specimens to evaluate d 13 C ontogenetic changes of each species. The data show no consistent patterns between size classes in the d 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 d 13 C ontogenetic changes exist within the >250 to >750 mm 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 d 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 d 13 C data; it is 0.27% for H. elegans at 3010 m and 0.4% 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. INDEX TERMS: 4870 Oceanography: Biological and Chemical: Stable isotopes; 1045 Geochemistry: Low-temperature geochemistry; 3030 Marine Geology and Geophysics: Micropaleontology; 4267 Oceanography: General: Paleoceanography; 4804 Oceanography: Biological and Chemical: Benthic processes/benthos; KEYWORDS: benthic, foraminifera, isotopes, benthos, paleoceanography, micropaleontology 1. Introduction [2] Oceanic circulation plays a critical role in global biogeochemical cycles and influences regional and global Copyright 2002 by the American Geophysical Union /02/2001PA climatic conditions. Changes in deep-ocean ventilation on various timescales, linked to heat transport in the oceans and the global CO 2 cycle, have been suggested to be an important factor influencing large-scale climatic changes. Many reconstructions of the strength and pathways of deepocean circulation have been based on time series of micropaleontological and chemical proxies from deep-sea 8-1

2 8-2 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Figure 1. Bathymetry of the northwest Atlantic Ocean in meters with the location of the sediment trap mooring at 3096 m and coring site at 3010 m indicated by a solid triangle and the coring site at 2430 m indicated by a solid circle. sediments. One geochemical proxy that has been widely used is the carbon stable isotopic composition of fossil benthic foraminifera from sediment sequences, which has provided estimates of the relative age and ventilation rates of deep waters [Rohling and Cooke, 1999]. Carbon isotope paleocirculation studies generally utilize epifaunal benthic foraminiferal taxa that live at the sediment/water interface in direct contact with the overlying bottom waters because these taxa are believed to most accurately reflect bottom water d 13 C. [3] The initial calibrations of benthic foraminiferal d 13 C and bottom water d 13 C, which formed the basis for the use of d 13 C in paleoceanographic reconstructions, were based on live plus dead specimens from core tops taken from throughout the oceans [Woodruff et al., 1980; Belanger et al., 1981; Graham et al., 1981; Grossman, 1984; Wefer and Berger, 1991]. The age of these fossils ranges from Recent to a few thousand years owing to mixing of specimens of varying age in the uppermost sediments as a result of bioturbation by macrobenthos. A more detailed understanding of the relationship between the biology of the organisms and the isotopic signal is based on d 13 C data of living specimens [Mackensen and Douglas, 1989; McCorkle et al., 1990, 1997; Rathburn et al., 1996]. Interspecific differences in d 13 C are attributed primarily to microhabitat effects between epifaunal and infaunal species, although vital effects due to physiological processes are also evident with some taxa [Rohling and Cooke, 1999]. [4] Recently, it has been suggested that seasonal differences in organic carbon flux may influence the carbon isotopic composition of epifaunal taxa owing to pulses of organic matter delivered to the seafloor following phytoplankton blooms ( phytodetritus events) [Mackensen et al., 1993]. The existing d 13 C data based on living specimens are from samples collected during a single cruise to any given site, and as a result, no information exists on seasonal variability of d 13 C in the foraminiferal tests. It is particularly important to determine if seasonal differences do exist because it is critical for the interpretation of d 13 C records used in paleocirculation reconstructions to understand how the benthic foraminiferal d 13 C signal is constructed and what factors control the variability of that signal. [5] To address this question, a study was carried out to determine seasonal changes over a 1 year interval of the biology and chemistry of benthic foraminifera at a deepwater study area ( m) 275 km south of Nantucket Island (Figure 1) in the northwest Atlantic Ocean. A sediment trap mooring with a Parflux Mark 7G 21-cup automated sediment trap was used to obtain an 11.5 month record of organic carbon flux, which is compared with benthic foraminiferal isotopic data based on Multicorer and Soutar Corer samples taken during five cruises (March, May, July, and October 1996 and January 1997] at two sites (2430 m: N, W; 3010 m: N, W). The mooring was located just south of the 3010 m site ( N, W) in 3096 m water depth.

3 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-3 The carbon isotopic data are compared with bottom water d 13 C data taken on each cruise from the coring sites. In this report, we assess the d 13 C seasonal record of Hoeglundina elegans, an epifaunal aragonitic species dominant at the 3010 m study site, and Uvigerina peregrina, an inferred infaunal species dominant at the 2430 m site. Data for Planulina wuellerstorfi are also reported, but no conclusions on seasonal variation can be reached because of the low number of analyses. 2. Previous Work 2.1. Phytodetritus Events and Deep-Sea Biology [6] The flux of organic carbon to the seafloor is highly variable and exhibits a seasonal pattern in most regions of the oceans. Changes in the amount of new production are related to upper ocean hydrographic conditions that create suitable conditions for episodic phytoplankton blooms. Most particulate organic carbon formed in the euphotic zone is recycled by organisms within the water column and only a small fraction (1 2%) reaches the seafloor [Suess, 1980; Wefer, 1989] in a matter of a few weeks following phytoplankton blooms [Billett et al., 1983; Lampitt, 1985; Hecker, 1990]. The small amount of organic carbon to reach the seafloor is either consumed by the benthos, providing the energy source for most deep-sea organisms [Smith et al., 1992], or is sequestered within deep-sea sediments. A large part (50 85%) of the organic material to reach the seafloor is thought to be remineralized within 1 year [Cole et al., 1987]. [7] Phytoplankton blooms in the surface waters are followed by the rapid flux ( m d 1 ) of phytoplankton and zooplankton remains that create a fluffy layer of phytodetritus on the seafloor [Billett et al., 1983; Rice et al., 1986; Thiel et al., 1989]. These phytodetritus events occur over wide areas of the ocean and have been observed with deep-sea photography and collected in Multicorer samples. The phytodetritus layers have a patchy distribution and are easily resuspended by bottom currents. Once on the seafloor, the phytodetritus layer serves as a food source and affects bacterial and meiobenthos standing stocks [Lochte and Turley, 1988; Gooday and Turley, 1990; Pfannkuche, 1993; Turley et al., 1995] and is remineralized over an interval of several months [Rice et al., 1994]. [8] The seasonal and variable input of organic carbon to the deep-sea environment [Honjo, 1996] has a dramatic effect on deep-sea benthic foraminifera with changes in species dominance, abundance, and rapid growth over a period of a few months [Gooday, 1988; Gooday and Lambshead, 1989; Gooday, 1993, 1996; Smart and Gooday, 1997; Kitazato and Ohga, 1995; Ohga and Kitazato, 1997; Gooday and Rathburn, 1999; Loubere and Fariduddin, 1999; Kitazato et al., 2000; Bernhard and Reimers, 1992; Corliss and Silva, 1993; Silva et al., 1996] Carbon 13 of Deep-Sea Benthic Foraminifera [9] The d 13 C of epifaunal species reflects the d 13 C gradient found in the bottom waters of the Atlantic and Pacific Oceans [Graham et al., 1981; Duplessy et al., 1984], which provides the basis for using carbon isotopes in paleocirculation studies. A number of geochemical studies of deep-sea benthic foraminifera have documented interspecific differences in d 13 C, however, which reflect disequilibria of some taxa with the overlying bottom water [Woodruff et al., 1980; Belanger et al., 1981; Grossman, 1984, 1987]. These interspecific differences were attributed to either vital effects or species living in different microenvironments. With the recognition that benthic foraminifera have species-specific microhabitat preferences within the upper cm of deep-sea surficial sediments [Corliss, 1985], a comparison of pore water d 13 C and shell composition of both infaunal and epifaunal taxa showed that a microhabitat effect does exist, although vital effects cannot be entirely ruled out for some taxa [McCorkle et al., 1990, 1997; Rathburn et al., 1996]. [10] Distributional studies of living (stained) benthic foraminifera suggest that H. elegans is an epifaunal species [Corliss, 1985; Corliss and Emerson, 1990]. Hoeglundina d 13 C values are consistently enriched by between 1 and 2% relative to bottom water dissolved inorganic carbon (DIC) [Graham et al., 1981; McCorkle et al., 1990, 1997; Rathburn et al., 1996; Sommer and Rye, 1978; Grossman, 1984, 1987], which is thought to reflect the equilibrium carbon isotope enrichment of aragonite [Romanek et al., 1992]. Bottom water temperature and Hoeglundina d 13 C were reported to vary inversely [Grossman, 1984, 1987], but since temperature varies with water depth and other ecological factors, it is difficult to assess the influence of temperature on the carbon isotopic data. Although H. elegans stable isotope data have not been widely used in paleoceanographic reconstructions, Cd/Ca data from this species have been utilized [Boyle et al., 1995; Boyle and Rosenthal, 1996]. Observations on the isotopic behavior of this species can provide information relevant to other biconvex epifaunal taxa. [11] Uvigerina peregrina is generally found in the upper 2 cm of surficial sediments [Corliss, 1985; Corliss and Emerson, 1990; Rathburn et al., 1996] and is inferred to be a shallow-dwelling infaunal species on the basis of its distributional pattern in the sediments and morphological features [Corliss, 1991]. Uvigerina has also been documented at deeper depths in the sediment and attributed to the presence of suitable microhabitats within the sediments due to biological activity [Corliss and van Weering, 1993; Loubere et al., 1995]. Uvigerina d 13 C values range from 0.5 to 2.0% lower than bottom water d 13 C values [Woodruff et al., 1980; Graham et al., 1981; Grossman, 1984, 1987; McCorkle et al., 1990,1997]; this depletion is thought to reflect calcification within the low d 13 C pore waters of the microhabitat that this species occupies in the surficial sediments [McCorkle et al., 1990, 1997]. Core top gradients in pore water d 13 C values reflect the flux of organic carbon to the seafloor, and we have suggested that the range of Uvigerina d 13 C values may thus reflect variations in the productivity of the surface ocean [McCorkle et al., 1990, 1997]. Planulina wuellerstorfi is an epifaunal species that prefers an epibenthic microhabitat [Lutze and Thiel, 1989], attached to rocks, spicules, etc., above the seafloor. This species typically has d 13 C values close to the d 13 C of bottom water dissolved inorganic carbon [Duplessy et al., 1984;

4 8-4 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA McCorkle and Keigwin, 1994], although negative offsets have been reported in highly productive regions [Sarnthein et al., 1988; Mackensen et al., 1993]. [12] These studies demonstrated that epifaunal taxa are closest to equilibrium of the overlying bottom water and therefore are the most reliable for reconstructing d 13 C conditions in the deep ocean. This view of the reliability of d 13 C of epifaunal taxa to reflect bottom water d 13 C was recently challenged by Mackensen et al. [1993] on the basis of core top data from the South Atlantic sector of the Southern Ocean. They suggested that the presence of phytodetritus could alter the d 13 C gradient at the sediment-water interface, which in turn, could affect the d 13 C of epifaunal species living in this microhabitat. 3. Study Area [13] The study area was chosen because of relatively high primary productivity of 300 g C m 2 yr 1 [O Reilly et al., 1978] and the seasonal presence of phytodetritus on the seafloor [Hecker, 1990]. The spring phytoplankton bloom in the northwest Atlantic in the vicinity of Georges Bank begins in late March early April and is generally finished by early May [Cura, 1978]. The timing of the cruises provided samples immediately before (March) and after (May) the spring phytoplankton bloom and at intervals during the remainder of the year (July, October, and January). Phytodetritus was previously observed slightly to the north (41 ) of the study area from 500 to 2500 m Table 1. Summary of Cruise Information Cruise Date Number of Subcores Comments EN279 February trap deployment EN281 March EN284 May OC283 July trap turnaround EN289 October EN293 January trap recovery in late April and early May 1985 and consisted of a fluffy layer covering much of the seafloor [Hecker, 1990], similar to what has been observed in the northeast Atlantic and elsewhere [Billett et al., 1983]. Phytodetritus consists primarily of phytoplankton cells (often mostly diatoms), bacteria, and gelatinous aggregates, with lesser amounts of small grazers (dinoflagellates, radiolaria, and foraminifera), and the fecal pellets of larger consumers. This material is transported rapidly from the surface to the seafloor ( m d 1 ). A fluffy green layer up to a few centimeters height blankets the seafloor for a few weeks to months following phytoplankton blooms. The fluff layer is easily resuspended and has a highly variable distribution on the seafloor. [14] Norwegian-Greenland Sea Overflow water, which is a component of North Atlantic Deep Water, is found overlying the seafloor from 2500 to 4000 m in the study area with bottom temperatures of 2 3 C [Pickart, 1992]. Salinities of % and dissolved oxygen values of Table 2. Core Sample Locations and Depths Latitude Longitude Depth Type EN Soutar Soutar Soutar EN MC MC MC MC MC MC MC MC OC MC MC MC MC MC EN Soutar MC MC EN MC MC MC

5 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-5 Table 3. Oxygen and Carbon Isotopic Data of Living (Rose Bengal Stained) H. elegans, U. peregrina, and P. wuellerstorfi From This Study a Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN281 EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN * EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN284 EN * EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN * EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans

6 8-6 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN EN H.elegans EN H.elegans EN H.elegans EN H.elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN * EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN * EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans

7 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-7 Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans OC283 OC OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans

8 8-8 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans OC H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans

9 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-9 Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN EN H. elegans EN293 EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H.elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN * EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN * EN H. elegans EN H. elegans EN H. elegans EN H. elegans EN H. elegans

10 8-10 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN * EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN >750 H. elegans EN281 EN EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U.peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN284 EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina

11 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-11 Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina

12 8-12 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN EN U. peregrina OC283 OC * OC U.peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC * OC U. peregrina OC U. peregrina OC U. peregrina OC * OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina

13 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-13 Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O OC U. peregrina OC U. peregrina OC U. peregrina OC * OC U. peregrina OC U. peregrina OC U. peregrina OC U. peregrina EN293 EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U.peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina

14 8-14 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN >750 U. peregrina EN * EN U. peregrina EN U. peregrina EN U. peregrina EN281 EN EN P. wuellerstorfi EN * EN P. wuellerstorfi EN P. wuellerstorfi EN284 EN * EN P. wuellerstorfi EN P. wuellerstorfi EN P.wuellerstorfi EN P. wuellerstorfi EN EN >750 P. wuellerstorfi EN >750 P. wuellerstorfi EN >750 P. wuellerstorfi EN >750 P. wuellerstorfi EN EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN >750 P. wuellerstorfi OC283 OC OC P. wuellerstorfi OC P. wuellerstorfi OC OC P. wuellerstorfi OC P. wuellerstorfi OC P. wuellerstorfi OC P. wuellerstorfi OC OC P. wuellerstorfi OC P. wuellerstorfi OC OC P. wuellerstorfi OC P. wuellerstorfi OC P. wuellerstorfi OC OC P. wuellerstorfi OC P. wuellerstorfi

15 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA 8-15 Table 3. (continued) Core, Subcore, and Interval (cm) Size, mm Species d 13 C d 18 O OC P. wuellerstorfi OC P. wuellerstorfi OC P. wuellerstorfi EN293 EN EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi EN P. wuellerstorfi a Cores marked with an asterisk are from 2430 m; all others are from 3010 m. >270 mmol kg 1 are associated with this water mass [Fuglister, 1960], which overlies fine-grained muds with silt contents of >50% and organic carbon values of % [Doyle et al., 1979]. [15] The first Shelf Edge Exchange Processes experiment (SEEP-I) consisted of an array of sediment traps on a northsouth transect at 71 W, slightly to the west of the study site [Biscaye et al., 1988]. The deepest mooring included a trap at 2700 m (50 m off the bottom) which showed distinct variations in total flux and organic carbon during the 14 month experiment. Although components of the organic flux were suggested to be exported from the continental shelf or transported downslope, the total mass fluxes measured in the sediment traps were representative of the longterm accumulation rates of the slope sediments [Biscaye et al., 1988], indicating that the trap data were reliable measures of the sedimentation in the area. This observation is particularly important in this area where the Deep Western Boundary Current can have an effect on sediment redistribution. 4. Methods [16] Six cruises were carried out over the course of 1 year (Tables 1 and 2) to obtain living (stained) benthic foraminifera and a time series of organic carbon flux. Benthic foraminifera were obtained with a Multicorer for all samples except those in March and one core in October when a Soutar Box Corer was used. We have found from past experience that the Soutar Corer obtains a sample with an undisturbed sediment-water interface [Corliss and Emerson, 1990; McCorkle et al., 1990] and used this corer when weather conditions precluded recovery of sediments with the Multicorer. Sediment samples were not obtained during the February 1996 cruise because of ship equipment problems. Ten cm diameter cores were subsampled at 0.5 cm intervals from 0 to 3 cm and at 1 cm intervals from 3 to 15 cm and stored in a buffered seawater solution with 4% formalin. This study is based on samples from the and 0 1 cm intervals. Samples from the deeper intervals are not analyzed because of time constraints but were archived for future study. Our assumption was that the largest impact of seasonal flux of organic carbon would be on the cm interval, rather than at deeper depths within the sediments. In addition, previous d 13 C data from live specimens show relatively constant values, regardless of the sample depth interval [McCorkle et al., 1997]. In the laboratory, Rose Bengal stain was added to the samples, and each sample was left for a minimum of 7 days before processing [Corliss and Emerson, 1990]. The sediment volume for each sample was determined by subtracting the amount of seawater, formalin, and stain added during processing from the total volume. [17] Stained specimens are recognized as those having bright red or violet coloration in at least one chamber. Specimens with a tint of pink color over the entire test reflecting an organic membrane, spots of red color due to the presence of particles of organic matter, or a thin ribbonlike structure reflecting a worm living in a dead foraminiferal shell were not counted. Rose Bengal is a protein stain that has been widely used in benthic ecological studies for the identification of live organisms. The method does have its limitations, however, since it has been shown to stain dead foraminifera up to 4 weeks after the death of an organism [Bernhard, 1988]. Because of this, we used a conservative interpretation of the staining technique by assuming that stained specimens reflect protoplasm-containing tests which are either alive at the time of collection or have been alive in the recent past. Rose Bengal stain is a suitable stain for this work since our previous work, as well as other studies [Gooday, 1988; Gooday and Lambshead, 1989; Silva et al., 1996; Bernhard and Reimers, 1992; Buzas, 1993], has used this stain and shown changes in foraminiferal standing stocks and species abundances over intervals of only a few months. [18] Once a sample has been stained, it is sieved over 63 and 150 mm sieves, and each size fraction is stored in a 4% formalin solution. The mm fraction is stored for future study; the >150 mm fraction is picked wet in a gridded petri dish, and the specimens are placed on a micropaleontological slide and sorted by species. In this paper, isotopic data are presented for H. elegans, dominant at the 3010 m site and common at the 2430 m site, and U. peregrina, dominant at the 2430 m site, as well as for P. wuellerstorfi from the 3010 m site. [19] The oxygen and carbon isotopic compositions of stained specimens (Table 3) were determined at the Woods Hole Oceanographic Institution in W. Curry s laboratory using a Finnigan MAT 252 Mass Spectrometer with a Kiel Carbonate Preparation Device. The precision for carbon is

16 8-16 CORLISS ET AL.: d 13 C TIME SERIES OF BENTHIC FORAMINIFERA Figure 2. Scanning electron micrographs of H. elegans and U. peregrina with the length measurement taken of each specimen indicated by the white bar. ±0.03%, and for oxygen it is ±0.08%. All isotopic values are reported as per mil differences from the Peedee belemnite (PDB) international standard. [20] All specimens were measured (Figure 2) and grouped into 100 mm size classes for isotopic analysis. The measurements were also used to assess changes in population structure during the 1 year interval, which will be reported elsewhere. Most isotopic measurements are based on the analysis of single specimens with a minimum weight of 15 mg; specimens >350 mmofh. elegans and >450 mmfor U. peregrina were all analyzed as single specimens, and data from smaller size classes are based on two to three specimens. [21] In order to estimate the standard deviations assigned to the various factors in the data for a given species a nested mixed effects model [Hocking, 1985] was fit to the data. This model treats the measured response as a sum of effects due to cruise t i, core c j(i), subcore s k( j) and noise e ijkl so that y ijkl = t i + c j(i) + s k( j) + e ijkl, where i indexes cruises, j indexes cores, k indexes subcores, and l indexes replicates within subcore. The notation j(i) and k( j) reminds us that the cores j are nested within cruises i, and the subcores k are nested within cores j. In addition, the core values are modeled as independent draws from a N(0, s c 2 ) distribution, the subcore values are modeled as independent draws from a N(0, s s 2 ) distribution, and the noise components e ijkl are modeled as independent draws from a N(0, s e 2 ) distribution. Variance estimates for the model were obtained using proc mixed in SAS, version 6.2 (See Littell et al. [1996, section 4.4] for an example). This procedure makes use of maximum likelihood, and therefore the likelihood ratio test is appropriate for testing for the presence of nonzero variance components. [22] The carbon isotopic composition of the bottom water was determined on samples obtained from a Niskin bottle attached to the frame of the corer and rigged to trip when the corer hit the bottom. Small samples (5 ml) were sealed into prepoisoned glass ampules immediately upon core recovery using methods developed for pore water sampling [McCorkle et al., 1995, 1997]. The dissolved inorganic carbon was extracted on shore, and the isotopic composition of the extracted CO 2 was determined on the VG-PRISM mass spectrometer at the National Ocean Sciences Accelerator Mass Spectrometer Facility at the Woods Hole Oceanographic Institution. The standard deviation of replicate bottom water samples (collection, extraction, and analysis) averaged ±0.05%. Figure 3. A record of organic carbon flux (mg m 2 d 1 ) at 3096 m from February 1996 to January Vertical lines indicate the time of the six cruises with the cruise numbers shown. Note the maxima in organic carbon flux in late April and from middle August through October. The trap was recovered and redeployed in July The record from February to July is based on an 8 day sampling interval and from July to January on an 8.5 day interval.