Seasonal changes in POC export flux in the Chukchi Sea and implications for water column-benthic coupling in Arctic shelves

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Deep-Sea Research II 52 (2005) 3427–3451 www.elsevier.com/locate/dsr2

Seasonal changes in POC export flux in the Chukchi Sea and implications for water column-benthic coupling in Arctic shelves S.B. Morana,, R.P. Kellya, K. Hagstroma, J.N. Smithb, J.M. Grebmeierc, L.W. Cooperc, G.F. Cotad,{, J.J. Walshe, N.R. Batesf, D.A. Hansellg, W. Maslowskih, R.P. Nelsonb, S. Mulsowi a Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, NS, Canada B2Y 4A2 c Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA d Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, VA 23508-0369, USA e College of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA f Bermuda Biological Station for Research, Ferry Reach, St George’s GE01, Bermuda g Rosentiel School of Marine and Atmospheric Research, University of Miami, Miami, FL 33149, USA h Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943, USA i Instituto Geociencias, Universidad Austral de Chile, PO Box 567, Valdivia, Chile

b

Received 5 March 2004; accepted 10 September 2005

Abstract As part of the 2002 Shelf-Basin Interactions (SBI) process study, measurements of the seasonal variation in the export flux of particulate organic carbon (POC) are reported for the upper waters of the Chukchi Sea. POC fluxes were quantified by determination of 234Th/238U disequilibrium and POC/234Th ratios in large ð453 mmÞ aggregates collected using in situ pumps. Samples were collected at 35 stations on two cruises, one in predominantly ice-coved conditions during the spring (May 6–June 15) and the other in predominantly open water during the summer (July 17–August 26). Enhanced particle export was observed in the shelf and slope waters, particularly within Barrow Canyon, and there was a marked increase in particle export at all stations during the summer (July–August) relative to the spring (May–June). 234Th-derived POC fluxes exhibit significant seasonal and spatial variability, averaging 2:9
5:3 mmol C m2 d1 ðrange ¼ 0:031222 mmol C m2 d1 Þ in the spring and increasing 4-fold in the summer to an average value of 10:5
9:3 mmol C m2 d1 ðrange ¼ 0:79239 mmol C m2 d1 Þ. The fraction of primary production exported from the upper waters increases from 15% in the spring to 32% in the summer. By comparison, DOC accumulation associated with net community production represented 6% of primary production ð2 mmol C m2 d1 Þ. The majority of shelf and slope stations indicate a close agreement between POC export and benthic C respiration in the spring, whereas there is an imbalance between POC export and benthic respiration in the summer. The implication is that up to 20% of summer

Corresponding author. Tel.: +1 401 874 6530; fax: +1 401 874 6811.

E-mail address: [email protected] (S.B. Moran). Deceased July 1, 2004.

{

0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.09.011

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production ð6
7 mmol C m2 d1 Þ may be seasonally exported off-shelf in this productive shelf/slope region of the Arctic Ocean. r 2005 Elsevier Ltd. All rights reserved. Keywords: Arctic; Carbon; Thorium-234; Scavenging; Particulate; Shelf

1. Introduction Ship-board, satellite, and modeling studies of the Arctic Ocean indicate marked seasonal, interannual and decadal changes in sea-ice coverage and thickness, physical forcing, circulation, water column geochemical properties, biological productivity and community structure (Carmack et al., 1997, 1995; Comiso et al., 2003; Laxon et al., 2003; Maslowski et al., 2001; McLaughlin et al., 2002; Walsh et al., 2004, 2005). Such changes are particularly evident in the Chukchi Sea, where rates of primary production are among the highest in the world ocean (Springer and McRoy, 1993; Walsh, 1989a,b), though more modest on the outer shelf and basin (Cota et al., 1996; Gosselin et al., 1997). Outstanding questions are what fraction of primary production is seasonally exported downward as sinking organic matter to the benthos, and how much of this organic matter is retained in shelf and slope sediments on seasonal and decadal timescales? These questions bear on our understanding of carbon and nutrient cycling, the efficiency of pelagic–benthic coupling, and the mechanisms for exchange of carbon and other chemicals (including contaminants) between the shelves and interior Arctic Ocean. A key issue is the seasonal variability in off-shelf particle export in the Arctic, which is likely to be significant because of the strong seasonal gradients in dissolved nutrient concentration, light intensity, and ice-cover. Past studies in the Canada Basin indicate a seasonal cycle in primary productivity and respiration under the permanent ice-cover (Sherr et al., 1998). These observations are consistent with sediment trap data that indicate a seasonal cycle in particle flux under the permanent ice-cover (Hargrave et al., 1994). Recent measurement of the particle-reactive radionuclides 230Th and 231Pa in the Canadian Basin (Edmonds et al., 1998, 2004; Trimble et al., 2005) also imply higher and more variable rates of scavenging and particle flux than previously measured over the Alpha Ridge (Bacon et al., 1989). Although the integrated annual particle export flux may not be high by world ocean

standards, on a seasonal basis, the export flux of particulate organic matter and associated reactive elements in the western Arctic may be comparable to more productive ocean basins (Wassmann et al., 2003). As part of the Shelf-Basin Interactions (SBI) Phase II process study, we present estimates of particulate organic carbon (POC) export flux determined from 234Th/238U disequilibrium along seven shelf-basin sections in the Chukchi Sea during spring and summer, 2002 (Fig. 1). 234Th ðhalf-life ¼ 24:1 daysÞ is a particle-reactive radionuclide produced continuously in the oceans by alpha decay of soluble 238U ðhalf-life ¼ 4:5  109 yearsÞ and is a sensitive tracer of scavenging and particle export occurring on a time-scale of days to months (Coale and Bruland, 1985; Matsumoto, 1975; Santschi et al., 1979). Seasonal changes in these POC export fluxes are considered in context with rates of primary productivity (Hill et al., 2005), benthic carbon respiration (Grebmeier et al., 2004), and

Fig. 1. Map of the Chukchi Sea and adjacent Canada Basin indicating sampling stations occupied for 234Th and POC sampling during the SBI-II process cruises in spring ðÞ, May 6–June 15, and summer, ðÞ July 15–August 26, 2002. Shelf-slope transects are designated as WHS (West of Hanna Shoal), EHS (East of Hanna Shoal), BC (Barrow Canyon), and EB (East of Barrow Canyon).

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dissolved organic carbon (DOC) accumulation (Mathis et al., 2005) measured on the same cruises to provide a synoptic view of the transport and fate of organic carbon within this shelf and slope region. 2. Methods 2.1. Sample collection and processing Water-column samples were collected from 35 stations and 171 discrete depths for 234Th determination (684 samples analyzed total) in the Chukchi Sea in May 6 to June 15 (HLY-02-01) and July 17 to August 26 (HLY-02-03), 2002, onboard the U.S.C.G.C. Healy (Fig. 1). Samples were collected along three shelf-slope transects in spring 2002, designated WHS (West of Hanna Shoal), EHS (East of Hanna Shoal), and BC (Barrow Canyon). These transects were reoccupied during the summer 2002 cruise, with the addition of the EB transect (East of Barrow Canyon). Large volume sampling (ca. 200–1000 l) was conducted using in situ pumps (Challenger Oceanic Systems and Services, U.K.) Discrete seawater samples were pumped sequentially through a 53-mm, 142 mm diameter, Nitex screen, a 1-mm pore-size, 10 in (25.4 cm) length, polypropylene prefilter cartridge and two 3.3-in (8.46 cm) MnO2 impregnated cartridges, connected in series to scavenge dissolved 234Th. During the cruise, both the prefilter and MnO2 cartridges were dried at 60 1C and then combusted in Pyrex beakers at 500 1C for 8–10 h in a vented muffle furnace located on the rear deck of the ship. The sample ash was then transferred from the beakers to counting jars for the planar detector or vials for the well detector. Unfiltered seawater samples (60 samples total, 250 ml each) were also collected from approximately 30 randomly spaced stations during each cruise for determination of 238U. Samples were stored in polyethylene bottles prior to ICP-MS analysis. Particulate matter collected on 53-mm Nitex screens was resuspended into GF/F filtered seawater by ultrasonication for 2–5 min (Charette et al., 2001). The resuspended particles were then collected by filtration through precombusted GF/F filters and stored frozen until 234Th and POC/PON analysis. The efficiency of large particle resuspension by ultrasonication has been previously reported to be approximately 90–95%, with 5–10% of particles remaining on the Nitex screen (Buesseler et al., 1998; Cochran et al., 2000). Because the 453-mm

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Th activity typically represents only o3% of the total 234Th activity, retention of large particle 234Th on Nitex screens after ultrasonication would not significantly affect the total 234Th activity and hence our calculation of the 234Th particle export flux ðPTh Þ. We further assume that the POC/234Th ratio is not altered during resuspension (Buesseler et al., 1998; Cochran et al., 2000). Seawater samples (16–23 l) were drawn from Niskin bottles into plastic containers for determination of suspended particle mass (SPM) during the spring cruise. Unfiltered seawater was passed through preweighed 47-mm diameter 0:45-mm Nuclepore filters using a peristaltic pump. Filters were rinsed with deionized water (pH 8) to remove sea salt. Samples were dried and stored frozen in plastic Petri dishes prior to analysis.

234

2.2.

234

Th and

238

U analyses

The activity of 234Th in the ashed prefilter and MnO2 cartridge samples was determined by gamma spectrometry using both ship-board and shore-based (at URI) pure Ge gamma detectors equipped with ultra low-background cryostats. The 63.3 keV peak was used to calculate the 234Th activity (Buesseler et al., 1992b; Charette et al., 2001; Moran et al., 1997a). All 234Th data were decay-corrected to the mid-point when each individual sample was collected. Shipboard gamma spectroscopy was conducted using a Canberra planar detector with a carbon-fiber window (2000 mm2 active area). Gamma counting at URI was conducted using a Canberra well-type detector (150 cm3 active volume). The fraction of the total ash analyzed was 95–98% for the planar detector (at sea) and 30–40% of the prefilter ash and 70–90% of the MnO2 ash for the well detector. The Ge detectors were calibrated for 234Th determination in the respective cartridge prefilter and MnO2 ash sample geometries using NIST 238U. The efficiency of the planar detector was 15
4% for both 4.5–5 g of MnO2 ash and 10–12 g of prefilter cartridge ash. The efficiency of the well detector was 48
2% for 3.5 g (4 mL) of MnO2 or prefilter cartridge ash. Dissolved ðo1 mmÞ 234Th activities were corrected for the collection efficiency ðEÞ of the MnO2 cartridges ðaverage ¼ 78
9%, N ¼ 171Þ, which was calculated using E ¼ 1  MnB=MnA, where MnA and MnB represent 234Th activities on the respective, sequential cartridges (Livingston and Cochran, 1987). The cartridge prefilter samples were assayed using

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gamma spectroscopy to within
10215% uncertainty, MnA samples to
527%, and MnB samples to
10215% (1-s counting uncertainty). The uncertainty in dissolved 234Th was propagated using the counting uncertainty and the uncertainty in the collection efficiency of the MnO2 cartridges; for total 234 Th, this includes the uncertainty in beta counting 453-mm particulate 234Th. The activity of particulate 234Th in samples collecting using 53-mm Nitex screen was determined by direct, non-destructive, beta counting using a RISØ National Laboratory low-background beta detector. After cutting a wedge for POC/PON analysis (20–30% by weight of the total filter), the 25-mm GF/F filters containing 453-mm particulate matter were wrapped in clear plastic ð1:5 mg cm2 ) and Al foil ð4:5 mg cm2 ) to minimize interference from background alpha and beta radiation. Samples were counted for 700 min (7  100 min intervals) once every two weeks for three cycles. These data were fitted to the 234Th decay curve and extrapolated back to the time of sample collection. The efficiency of the beta detectors for direct, non-destructive counting of GF/F filters was 35
5%, based on analysis of NIST 238U added to GF/F filters. This value is consistent with a beta detector efficiency of 34
4%, as determined by radiochemical purification, electroplating onto stainless steel disks, and beta counting (60
2% efficiency) of GF/F filter samples containing particulate 234Th that were previously analyzed by direct beta counting. The 453-mm particulate samples were counted long enough to achieve a
527% uncertainty. 238 U activities in the water column were calculated from salinity using a calibrated CTD according to the relationship 238 U ¼ 0:069  salinity (Ku et al., 1977). This relationship was checked by analysis of seawater 238U using a Finnigan MAT Element sector-inductively coupled plasma mass spectrometer (ICP-MS) operated in medium resolution mode with a Cetac MCN-6000 desolvation nebulizer and Tm as an internal standard. 2.3. Analysis of particulate organic carbon, nitrogen and suspended particle mass In the shore-based lab at URI, the 25-mm GF/F filter wedge subsamples containing 453-mm particulate matter were thawed, acidified in a desiccator with fuming concentrated HC1 for 24 h to remove inorganic carbon, and then dried overnight at 60 1C. POC and particulate nitrogen were quantified using a

Carlo-Erba CHN analyzer (Pike and Moran, 1997). The total mass of carbon and nitrogen per filter was corrected for the filter blank, converted to molar units, and divided by the sample volume filtered. Filter blanks were prepared by filtering 100–200 ml of GF/F prefiltered seawater and therefore represent the total blank associated with the precombusted GF/F filter and any DOC adsorption from ambient seawater (Moran et al., 1999). For the spring cruise, GF/F filter blanks were 6:61
1:69 mmol C and 1:11
0:66 mmol N per 25-mm filter. For the summer cruise, filter blanks were 4:46
3:94 mmol C and 1:09
0:74 mmol N per 25-mm filter. The 426 mmol C per 25 mm GF/F filter is consistent with previously reported filter blanks that include the contribution from DOC adsorption, which was determined empirically by successive filtration of varying volumes of seawater and extrapolation to zero water filtered (Moran et al., 1999). The preweighed 0:45-mm Nuclepore filters containing suspended particulate matter were dried to a constant weight and the mass determined gravimetrically. 3. Results 3.1. Hydrography, ice-cover, and euphotic zone properties The shelf and slope waters of the stations occupied in the Chukchi Sea are influenced by a 0:8 Sv in-flow through Bering Strait of nutrientrich Pacific Water (Roach et al., 1995). The pathways of Pacific Water into the Arctic Ocean extend northward from the Bering Strait via three distinct branches: western, central and coastal (Steele et al., 2004; Weingartner et al., 1998; Woodgate et al., 2005). The highly productive western branch of Pacific Water consists of a primary flow through Herald Valley, with some water continuing west into the Long Strait and possibly recirculating east around Wrangel Island. This branch is modified by a cold, fresh intermittent counter-flow, the eastward Siberian Coastal Current, that is present seasonally on an intermittent basis (Weingartner et al., 1999). On the eastern side, the Alaskan Coastal Current transports less productive, cold, fresh water from the Bering Sea towards the Barrow Canyon. The middle branch of Pacific Water enters the Central Channel between Herald Shoal and the Alaskan coast. On reaching the outer shelf, in the absence of wind and bottom friction, geostrophic constraints

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3.2.

234

Th and

238

U activities

234

Th and 238U activities determined for stations occupied in the Chukchi Sea are listed in Table A1. Results from the direct analysis of 238U in unfiltered seawater samples by sector-ICPMS are consistent with the calculated 238U activities over a wide range in salinity (Fig. 2). Total 234Th/238U activity ratios have been plotted using Ocean Data ViewR data analysis software (Schlitzer, 2002) (Figs. 3, 4). In the spring, 234Th/238U disequilibrium was highest at the shelf edge on the sections west and east of Hanna Shoal (WHS and EHS) and along the axis of Barrow Canyon (BC) (Fig. 3). In the interior waters 30250 km beyond the shelf break at WHS and EHS, total 234Th/238U activity ratios increase to values approaching secular equilibrium. During the summer, total 234Th/238U activity ratios were considerably lower, ranging from 0.4 to 0.8 in the upper water column (Fig. 4). Also, the partitioning of total 234Th between particulate ð41 mmÞ and dissolved forms indicates a seasonal shift from predominantly dissolved 234Th in the spring to an increasing fraction of particulate 234Th in the summer (Table A1). The range in total 234Th/238U disequilibrium and 234 Th activity is comparable to previous results reported for the Chukchi Sea shelf and slope waters, the Beaufort Sea, and the Canada Basin (Baskaran et al., 2003; Chen et al., 2003; Moran et al., 1997a, b; Moran and Smith, 2000; Trimble and Baskaran, 2005).

2.6

U (dpm L-1)

2.4

238

are thought to drive the Chukchi outflows into an eastward boundary current that flows along the shelf-break and slope towards the Beaufort Sea (Aagaard, 1984; Johnson, 1989; Pickart, 2004). There is evidence also of Pacific Water north of the Chukchi slope (Swift et al., 1997), suggesting that some of the Chukchi out-flow moves northward off the slope. Several studies (Mu¨nchow et al., 2000; Shimada and Carmack, 2001; Sverdrup, 1929) have reported a westward flow over the outer shelf or upper slope towards the Northwind Ridge and Chukchi Plateau. There is also significant eddy formation associated with the boundary flow, which regularly propagates relatively nutrient-rich shelf water into the interior basin (Pickart, 2004). Along the slope and below the upper waters of pure Pacific origin lies a well-defined halocline, with colder waters extending down to 200 m. At greater depths ð4200 mÞ, warmer low-nutrient Atlantic water is dominant, evident at the deeper interior stations (Swift et al., 1997). Shelf-basin interactions along the outer shelves and slopes of the Chukchi and Beaufort seas define the rates of exchange between Pacific Water and interior Arctic water masses. These processes contribute to the maintenance of the Arctic cold halocline (Aagaard et al., 1981), a well-defined hydrographic feature with cold waters extending down to 150–200 m. The cold halocline prevents melting of the multiyear ice pack by the underlying warm and low-nutrient Atlantic Water (Swift et al., 1997), which is distributed throughout the Arctic Ocean via boundary (slope) currents. During the 2002 SBI process cruises, surface waters had significant ice cover for all stations occupied in the spring, whereas the shelf and slope waters were largely ice-free at the stations occupied during the summer. Temperature and salinity in the upper 50 m ranged from 1:8 to 11C and 29–33% in the spring to 1:8 to 0.2 1C and 18–35% in the summer. The euphotic zone, defined as the 1% light level, averaged 37
14 m in the spring and 38
13 m in the summer (Hill et al., 2005). Euphotic depths were shallower ð25250 mÞ over the shelf and along the high productivity BC and EB sections in the summer (Hill et al., 2005). Chlorophyll-a concentrations increased from 0:120:3 mg m3 in the spring to 0:524 mg m3 in the summer (Hill et al., 2005). Dissolved NO3 ranged from 1 to 4 mM in shelf surface waters in the spring, whereas surface water concentrations were essentially undetectable during the summer.

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2.2 2.0 1.8 1.6

26

28

30 32 Salinity (‰)

34

36

Fig. 2. 238U activity measured by sector-ICPMS in unfiltered seawater plotted against salinity in samples collected from the Chukchi Sea, 2002. Dashed line is 238U activity calculated using 238 U ¼ salinity  0:069 (Ku et al., 1977).

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Fig. 3. Total

234

Th/238U activity ratios (AR) for sections occupied in the spring 2002 SBI process cruise in the Chukchi Sea.

3.3. Suspended particle mass The distributions of total SPM measured along the WHS, EHS and BC transects in the spring are characterized by low concentrations ð0:0520:1 mg l1 Þ in the upper 50 m underlain by much higher values ð0:1520:6 mg l1 Þ between 50 and 200 m that extend offshore from the shelf break (Fig. 5). The highest concentrations range from 0.5 to 1 mg l1 at 100–200 m in the shelf waters of the BC transect, which are approximately 2 orders of magnitude higher than the interior deep waters. While these concentrations are high, they are comparable to recent results reported for slope

waters in the Canada Basin (Trimble and Baskaran, 2005; Trimble et al., 2005). Further offshore and in the deep interior slope waters, SPM concentrations decrease to values of 0:0120:03 mg l1 , which approach values of 0:005 mg l1 reported for the central Arctic (Bacon et al., 1989). 4. Discussion 4.1. 34Th as an indicator of particle scavenging and export in the upper ocean Seasonal changes in particle scavenging and export are clearly evident from the cross-shelf and

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Fig. 4. Total

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234

Th/238U activity ratios (AR) for sections occupied during the summer 2002 SBI process cruise in the Chukchi Sea.

cross-slope sections of the total 234Th/238U activity ratio (Figs. 3, 4). The distribution of the total 234 Th/238U activity ratio in the spring is attributed to: (1) scavenging of 234Th by biogenic particles

associated with elevated rates of primary productivity in surface waters ðo50 mÞ near the shelf/slope break (Hill et al., 2005); and (2) particulate matter resuspended from shelf sediments, which is clearly

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Fig. 5. Total suspended particulate matter (SPM) concentration for the sections occupied during the spring 2002 SBI process cruise in the Chukchi Sea.

evident by high SPM concentrations in the subsurface waters proximal to the shelf break (Fig. 5). Higher activity ratios in the offshore waters are consistent with lower rates of primary productivity and associated particle export and the lack of shelf scavenging. In contrast, total 234Th/238U activity ratios observed in the summer indicate scavenging removal of 20260% of total 234Th from the shelfslope break and interior waters (Fig. 4). Removal of total 234Th from the upper waters coincided with a significant increase in primary productivity (Hill et al., 2005) and surface-water POC concentrations (Bates et al., 2005) during the summer cruise. The low total 234Th/238U activity ratios in the subsurface waters of all three sections in the spring

indicate effective shelf scavenging of 234Th (Fig. 3). This shelf-scavenging feature is centered at 100 m and extends 30 km from the shelf-break into the interior waters at WHS and EHS and along much of the axis of the BC transect, where 234Th/238U activity ratios range from 0.5 to 0.7 below 100 m (Fig. 3). Furthermore, the low 234Th/238U activity ratios in the subsurface waters over the shelf and shelf break coincide with the distributions of SPM (Fig. 5). The low total 234Th/238U activity ratios observed in the subsurface waters are attributed to enhanced scavenging of 234Th from water in close contact with the shelf and subsequent off-shelf transport into the interior waters on a time-scale of the mean-life of 234 Th (38 days). Due to its short half-life, 234Th

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would reach secular equilibrium with 238U during transit over a length scale of 100 km, extending from the shelves into the interior basin. This observation and interpretation is consistent with previous studies conducted in the Chukchi Sea (Moran et al., 1997a) and Beaufort Sea (Moran and Smith, 2000). A similar explanation has recently been reported to account for distributions of 210 Pb/226Ra disequilibrium in the western Arctic Ocean (Smith et al., 2003). 4.2. Export flux of POC estimated using disequilibrium

234

Th/238U

234

The Th-derived export flux of POC ðPPOC Þ is defined as the product of the POC/234Th ratio of sinking particulate matter times the depth integrated 234Th/238U disequilibrium in the upper ocean (Moran et al., 2003, and references therein),
 Z z C POC PPOC ¼ ðAU  ATh Þ dz, (1)  l ApTh 0 ApTh represent 1

where C POC and the concentration of organic C ðmmol l Þ and the 234Th activity ðdpm l1 Þ of sinking particulate matter, respectively, AU is the 238U activity ðdpm l1 Þ, ATh is the total 234 Th activity ðdpm l1 Þ, and l is the 234Th decay constant ð0:0288 d1 Þ. The equation assumes a steady-state and that transport of 234Th by advection and diffusion is small compared to the particle export flux of 234Th (Buesseler et al., 1992a, 1995; Charette et al., 2001). The 234Th method is an empirical technique, whereby measurements of the 234Th/238U disequilibrium and the POC/234Th ratio of sinking particles can be used to estimate the downward flux of POC from the upper waters. Over the past decade, 234Th has been increasingly used as a tracer of the vertical export flux of POC from the upper waters of the world ocean (Moran et al., 2003) and, more specific to the present study, within the Arctic Ocean and adjacent seas (Wassmann et al., 2003). Although this method has been widely used, a mathematical derivation of Eq. (1) has not been previously reported. Here, we outline the underlying assumptions of Eq. (1) by first defining the sinking flux of particulate 234Th at depth z as, X PTh ¼ ðAiTh  S iTh Þ, (2) i

where PTh represents the sinking flux of 234Th ðdpm m2 d1 Þ, i is the particle size, AiTh is the 234Th

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activity on particle size i ðdpm m3 Þ, and S iTh is the settling velocity ðm d1 Þ of particle size i containing 234 Th. If we substitute AiTh ¼ X iTh ApTh , where X iTh is the fraction of particulate 234Th ðApTh Þ on particle size i then, X PTh ¼ ApTh ðX iTh  SiTh Þ. (3) i

Similarly, an equation for the POC flux ðPPOC , mmol m2 d1 Þ at depth z can be written as, X PPOC ¼ C POC ðX iPOC  S iPOC Þ, (4) i

where C POC is the organic C concentration ðm mol m3 Þ of sinking particulate matter, X iPOC is the fraction of organic C of particle size i, and SiPOC is the settling velocity of particle size i containing POC. Finally, if, and only if, X X ðX iTh  S iTh Þ ¼ ðX iPOC  SiPOC Þ (5) i

i

then combining (3), (4), and (5),
 C POC PPOC ¼  PTh ApTh

(6)

and Eq. (6) is equivalent to Eq. (1). This also demonstrates that the 234Th flux ðPTh Þ calculated from the integrated 234Th deficit (Eq. (1)) over depth range z0  z (where, z0 ¼ 0 mÞ is equivalent to the flux of 234Th calculated by measurement of the 234 Th activity of sinking particles at depth z multiplied by the particle sinking speed (Eq. (2)). The key assumptions are that the particle sizedistribution of 234Th and POC and the sinking speed of particles that transport 234Th and POC out of the upper ocean are equivalent (Eq. (5)). This may be invalid due to differences in the respective source terms and particle-size distributions, rates of seawater-particle exchange (e.g., remineralization, adsorption), and different particle sinking speeds of POC and 234Th in the ocean. Furthermore, this assumption is also inconsistent with numerous studies indicating that the POC/234Th ratio varies with depth (Moran et al., 2003). The reason that the 234Th-technique can be applied, at least in principle, is that the methods used to sample the POC/234Th ratio (i.e., particle filtration, sediment traps) provide an integration of organic C and 234Th over all size ranges of sinking particles. Thus, the measured POC/234Th ratio for large particles of size i (e.g., 453 mm) is assumed to be representative of the total POC/234Th ratio of

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sinking particles as written in Eq. (6);

 C POC C POC ¼ . ApTh i453 mm ApTh

(7)

The above derivation provides a quantitative explanation as to why the different techniques used to sample POC/234Th ratios are critical to the accurate estimation of POC fluxes based on 234 Th/238U disequilibrium. 4.3. POC/234Th ratios Distributions of the POC/234Th ratio in 453 mm aggregates indicate maximum values of 10240 mmol dpm1 within the upper 25250 m in the spring (Fig. 6) and summer (Fig. 7). The highest ratios were observed in the summer on the BC and EB transects, which had the highest rates of primary productivity (Hill et al., 2005) and high POC/234Th ratios extending down to 100–150 m. The transition from high to low POC/234Th ratios in the upper waters coincides with the maximum in suspended and large aggregate POC concentration in the upper 50 m (Bates et al., 2005), The lowest POC/234Th ratios of 125 mmol dpm1 occur in the deep interior waters, which have lower particle abundances, POC concentrations, and particulate 234 Th activities. In addition, high POC/234Th ratios are evident throughout the water column over the shelf at WHS and BC in the spring and at BC and EB during the summer. The large-scale feature of a decrease in the POC/234Th ratio with depth observed in the Chukchi Sea (Figs. 6, 7) has been reported for the central Arctic (Moran et al., 1997a), the Beaufort Sea and Canada Basin (Baskaran et al., 2003; Chen et al., 2003; Moran and Smith, 2000), and a number of other oceanic regimes (Moran et al., 2003). The distributions of the POC/234Th ratio determined in this study are consistent with the preferential remineralization of POC and retention of 234Th on particles and aggregates as they sink and age through the water column (Bacon et al., 1996; Buesseler et al., 1995; Burd et al., 2000; Charette et al., 1999; Moran et al., 2003; Murray et al., 1996). There are few published measurements of large particle POC/234Th ratios collected by filtration within the Arctic Ocean proper, and only one detailed study has reported such data in a shelfslope environment, the Gulf of Maine-Scotian Shelf

(Charette et al., 2001). In the North Water (NOW) polynya, POC/234Th ratios in 470 mm aggregates collected by filtration ranged from 5 to 50 mmol dpm1 , similar to results in this study, and within a factor of 2–5 of POC/234Th ratios collected by floating sediment traps (Amiel et al., 2002). A more recent comparison of POC/234Th ratios in particles collected by large volume filtration ð453 mmÞ and floating sediment traps in the Barents Sea were within a factor of 2–3, ranging from 5 to 45 mmol dpm1 (Lalande et al., 2004). These results are also within the range of POC/234Th ratios collected by sediment trap in the Barents Sea of 329 mmol dpm1 (Coppola et al., 2002). The implication is that the 453-mm POC/234Th ratios collected by filtration in this study are likely to be representative of sinking particulate matter, at least to within a factor of 2, though further direct comparison of POC/234Th ratios collected by filtration and sediment trap is warranted. 4.4. POC export fluxes POC export fluxes were calculated for the upper 50 m of the water column, which, on average, is slightly deeper than the base of the euphotic zone (1% light layer) (Hill et al., 2005). The particle 234Th flux was calculated by trapezoidal integration of individual profiles of total 234Th activity over the 0–50 m depth range for all stations. The depthintegrated 234Th flux was then multiplied by the POC/234Th ratio measured in 453 mm aggregates at 50 m (Eq. (1)). When not measured directly, POC/234Th ratios at 50 m were interpolated using measurements immediately above and below this depth. The calculated POC fluxes therefore represent the flux of POC through the 50-m depth horizon. Because this depth corresponds to the base of euphotic zone, we report these results as POC export fluxes. In choosing a POC/234Th ratio and 234Th integration depth to calculate the POC export flux, it is important to consider, especially for the shelf waters, scavenging of 234Th by biogenic particle production and export as well as by abiogenic particles resuspended from the bottom. The close correspondence between the distributions of the total 234Th/238U activity ratio (Fig. 3) and SPM concentration in the spring (Fig. 5) indicates some influence of bottom resuspension on the total 234 Th/238U activity ratio. However, this influence is restricted to the subsurface waters (Fig. 5), in

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Fig. 6. POC/234Th ratios in 453-mm particles for sections occupied during the spring 2002 SBI process cruise in the Chukchi Sea.

most cases below 50 m, and therefore has only a minimal impact on the calculated 234Th deficit and POC/234Th ratio at this depth horizon for the shelf waters. Moreover, the depth of the euphotic zone averaged from 40
15 m (Hill et al., 2005). In addition, in the summer, sampling was conducted during bloom conditions under minimal ice coverage, which should maximize the production and export of fresh biogenic particulate matter. The assumption of a steady-state in calculating the POC export fluxes may result in an underestimate of the true flux when scavenging and particle export are increasing, for example, during bloom events (Buesseler et al., 1992a; Moran and Buesseler, 1993). In this study, sample collection

was conducted during pre-bloom conditions in the spring and when production was increasing in the summer, as evident from the total 234Th/238U activity ratios (Figs. 3, 4) and nutrients and biomass measurements (Hill et al., 2005). While there is clearly a time-varying seasonal change in primary production and particle flux, we do not have sufficient time-series data to determine the magnitude of non-steady-state changes in particle export over the duration of our sampling. The use of a 1-D model (Eq. (1)) may also underestimate the vertical 234 Th particle flux, due to exclusion of horizontal advection of 234Th. Previous studies in coastal environments indicate that inclusion of the horizontal advection term increased the vertical 234Th

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S.B. Moran et al. / Deep-Sea Research II 52 (2005) 3427–3451

Fig. 7. POC/234Th ratios in 453-mm particles for sections occupied during the summer 2002 SBI process cruise in the Chukchi Sea.

flux on average from 10% (Charette et al., 2001) to more than 30% (Gustafsson et al., 1998). POC export fluxes calculated for the SBI process cruises indicate significant seasonal and spatial variability throughout the Chukchi Sea (Fig. 8).

The average total POC export flux increased from 2:9
5:3 in the spring to 10:5
9:3 in the summer (Table 1). These results largely bracket the few previous measurements of 234Th-derived POC export flux reported for the Arctic Ocean and adjacent

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3439

Fig. 8. Spatial distribution of 234Th-derived POC export flux. Results from the spring (HLY-01-01) and summer (HLY-01-03) 2002 SBI-II cruises are plotted together with previous data from this region. All fluxes are reported at 50 m water depth, except Chen et al. (2003) at 100 m depth.

seas (Table 2, Fig. 8). Results indicate the highest fluxes over the shelf and along the shelf-slope break, particularly along Barrow Canyon, compared to the interior slope/basin waters. In the shelf waters ðo300 mÞ, POC export fluxes average 2:8
3:2 mmol C m2 d1 in the spring and 13:2
13:0 mmol C m2 d1 in the summer (Table 3). In the slope waters ð4300 mÞ, values average 1:3
1:6 mmol C m2 d1 in the spring and 8:8
6:0 mmol C m2 d1 in the summer. Taken as a whole, results from the SBI process cruises provide evidence for a 4-fold increase in the average total POC export flux on a time-scale of months. 4.5. Organic carbon balance and implications for pelagic– benthic coupling in Arctic shelves An important goal of the 2002 SBI process study was to evaluate the significance of shelf-slope export of POC on a seasonal time-scale. In this regard, we evaluated the relationship between POC export flux and rates of primary production (Fig. 9) and benthic C respiration (Fig. 10) measured during the SBI process cruises. Data are plotted only for those stations where all measurements were determined simultaneously. Rates of primary productivity were determined by 14C incubation (V. Hill, pers. comm.). Total sediment O2 utilization rates for

macro-, meio-, and microfauna (microbes), as well as chemical cycling, are mean rates from shipboard incubations conducted at in situ temperatures (Grebmeier et al., 2004). These were converted to C respiration rate units using a Redfield ratio 122 mol of C respired per 175 mol of O2 consumed (Pilson, 1998). This ratio assumes that most sediment respiration is due to macro benthic fauna and that rates of microbial anoxic respiration are low. This assumption is reasonable for the shelf and upper slope, although less applicable to lower slope and basin sediments where benthic fauna abundance is low. Note also that our sediment O2 uptake data represent a minimum value for sediment carbon mineralization because they do not include the contribution from anaerobic processes that accompany sediment carbon mineralization. There is a marked seasonal change in the magnitude and fraction of primary production exported from the upper waters (Fig. 9). The per cent ratio of POC export flux to primary productivity, defined as the Th–E ratio (Buesseler, 1998), increased from an average of 15% in the spring to 32% in the summer. The lower Th–E ratios are consistent with the pre-bloom conditions during the predominantly ice-covered spring season, characterized by elevated nutrient-low chlorophyll concentrations and low productivity. The shelf locations of

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Table 1 234 Th fluxes, POC/234Th ratios and POC export fluxes at 50 m in the Chukchi Sea, 2002 Station

Lat.  N

Spring, May 6– June 15, 2002 6-WHS2 72.90 7-WHS3 73.04 8-WHS4 73.26 9-WHS5 73.36 10-WHS6 73.44 11-WHS7 73.78 12-EHS11 73.44 14-EHS9 73.09 16-EHS7 72.87 18-EHS5 72.74 19-EHS4 72.60 24-BC2A 71.82 31-BC4 71.93 32-BC5 72.11 33-BC6 72.18 34-BC7 72.56 37-BC3 71.65

Long.  W

Water depth (m)

234

Th flux ðdpm m2 d1 Þ

POC/234Th453 mm ðmmol dpm1 Þ

POC flux ðmmol m2 d1 Þ

168.80 160.43 160.10 160.38 159.80 159.05 157.54 158.18 158.36 158.63 158.94 155.74 154.85 154.49 154.27 154.61 155.76

60 176 889 1190 1872 3071 2878 2108 970 236 83 100 548 1551 1849 2933 190

21117317 13347213 340758 5279 220733 7917126 255743 467784 393759 198732 7057120 337761 518778 6797109 298751 2775 7217130

3.670.2 3.570.2 4.170.3 0.670.1 1.370.1 1.970.2 4.170.2 3.170.2 2.870.2 5.970.4 0.770.1 0.770.1 11.170.7 1.070.1 2.070.2 5.370.4 30.172.1

7.671.2 4.770.8 1.470.3 0.0370.1 0.370.1 1.570.3 1.070.2 1.470.3 1.170.2 1.270.2 0.570.1 0.270.1 5.770.9 0.770.1 0.670.1 0.170.1 21.774.2

118 182 601 2235 2193 2306 406 197 85 158 245 403 978 2024 2264 1108 485 136

20897313 14077225 12487212 11517207 11687175 10637170 9977170 13927251 11847178 9227147 11837201 11347204 11157167 11817189 231739 7587136 9167156 11297203

18.771.1 14.871.0 9.170.7 16.071.0 10.870.8 9.370.7 10.270.6 8.170.6 8.670.7 4.270.3 1.870.1 4.270.3 4.570.3 15.371.1 3.470.3 3.570.2 3.970.3 4.470.4

39.176.3 20.873.6 11.372.1 18.473.0 12.672.2 9.971.9 10.171.6 11.272.0 10.271.9 3.970.6 2.170.4 4.870.9 5.070.8 18.173.2 0.870.1 2.670.4 3.670.6 5.070.9

Average POC flux ¼ 2:9
5:3 ðmmol m2 d1 Þ Summer, July 17– August 26, 2002 11-BC2 71.39 157.50 13-BC3 71.62 155.97 14-BC4 71.92 154.89 16-BC6 72.24 153.90 19-EB6 71.95 152.08 20-EB5 71.68 152.11 21-EB4 71.64 152.37 22-EB3 71.57 152.33 23-EB2 71.45 152.55 26-EHS4 72.65 158.62 27-EHS5 72.69 158.59 28-EHS6 72.83 159.33 29-EHS7 72.89 158.36 30-EHS9 73.06 157.92 33-WHS6 73.50 159.47 34-WHS5 73.30 160.24 35-WHS4 73.23 160.13 38-WHS2 73.01 160.65 Average POC flux ¼ 10:5
9:3 ðmmol m2 d1 Þ

the three stations distinguished by high carbon export in the spring suggest that these results may have been biased by an increase in the 234Th deficit due to shelf scavenging. In contrast, export production increased significantly at most stations in the summer, ranging from 10% to 60% of primary production (Fig. 9). The larger degree of scatter in the summer data may reflect the increased ‘‘patchiness’’ in rates of primary and export production during the productive summer months.

Results from the majority of shelf and slope stations occupied in the spring exhibit a good agreement between POC export and benthic C respiration rates (Fig. 10), particularly at shallower depths, where there was a tighter pelagic–benthic coupling of production. The unusually high POC export flux of 22:6 mmol C m2 d1 measured at Station 37-BC3 (Table 1) represents the last measurement made in the spring, and probably signals the onset of the summer bloom at this

ARTICLE IN PRESS S.B. Moran et al. / Deep-Sea Research II 52 (2005) 3427–3451 Table 2 Comparison of

3441

234

Th-derived POC fluxes in the Arctic Ocean and adjacent seas

Region

Export Depth (m)

POC/234Th ðmmol dpm1 Þ

PTh ðdpm m2 d1 Þ

POC flux ðmmol C m2 d1 Þ

Reference

Chukchi Sea, spring 2002 Chukchi Sea, summer 2002 Northeast Water Polynya Chukchi Shelf Central Arctic Ocean Beaufort Sea and Canada Basin North Water Polynya Barents Sea Canada Basin Bering Sea Canada Basin and Western Arctic Canada Basin and Western Arctic

50 50 50 30 30 50 100 50 100 100 50 50

0.6–30 1.8–19 13–120 67 5–21 8.5–12 5.7–8 3.4–9.2 1.2 7.0–9.4 8.3–9.8 6.2–37

27–2111 231–2089 262–966 550 55–420 75–625 800–2400 614–1549 824 1373–1648 894–902 129–1018

0.1–23 0.8–39 12–45 38 0.3–7 0.7–7 9–27 7–26 1 9.9–14 7.4–12.7 1.4–57

This study This study Cochran et al. (1995) Moran et al. (1997a,b) Moran et al. (1997a,b) Moran et al. (2000) Amiel et al. (2002) Coppola et al. (2002) Chen et al. (2003) Chen et al. (2003) Baskaran et al. (2003) Trimble and Baskaran (2005)

Table 3 Seasonal changes in organic C fluxes in the Chukchi shelf and slope, 2002 Flux

Spring

Flux

ðmmol C m2 d1 Þ

% PP

Shelfa PP POC exportb Benthic C respiration POC export—benthic

15:1
7:9 2:8
3:2 3:3
1:5 0:5
2:4

– 19 22 0

Slope PP POC export Benthic C respiration POC export—benthic

8:9
4:1 1:3
1:6 1:5
1:0 0:4
1:3

Total PP POC exportb Benthic C respiration POC export—benthic

12:0
6:9 1:8
2:2 2:3
1:5 0:5
1:9

Summer ðmmol C m2 d1 Þ

% PP

PP POC export Benthic C respiration POC export—benthic DOC accumulation

35.9723.8 13.2713.0 7.276.2 6.079.6 2.371.6

– 37 20 17 6

– 15 17 0

PP POC export Benthic C respiration POC export—benthic DOC accumulation

31.6721.9 8.876.0 2.772.3 6.174.2 1.771.1

– 28 9 19 5

– 15 19 0

PP POC export Benthic C respiration POC export—benthic DOC accumulation

32.9721.9 10.579.3 4.775.0 5.877.2 1.971.3

– 32 14 18 6

All fluxes are averages. a Shelf stations defined as o300 m. b High value at St. 37-BC3 not included (see text).

location. However, data from several stations occupied in the summer indicate an imbalance between POC export and benthic respiration. Some of the values that exhibit the poorest agreement are from stations with water depth 4400 m (Fig. 10). Benthic C respiration rates may be relatively low at these deep-water stations because the pulse of

freshly produced organic matter exported from the upper waters and/or from the off-shelf transport of carbon may not have reached the sediment/water interface by the time of benthic sampling. In addition, POC export fluxes may be inordinately high at some deep-water stations because not all sinking organic C is efficiently respired by the

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Spring Summer

25

Th-E ratio = 50%

-2

-1

POC Export Flux (mmol C m d )

30

20 15 10 10% 5 2% 0

0

10

20

30

40

50

60

70

Primary Productivity (mmol C m

-2

80

-1

d )

Fig. 9. POC export flux plotted against primary productivity indicating a marked seasonal increase in the fraction (Th-E ratio) of primary production exported by sinking particles.

-1

Summer

Benthic C Respiration (mmol C m d )

Spring

-2

20

1:1 15

10 1:2 190 m

5

0

0

5

10

15

20 -2

25 -1

POC Export Flux (mmol C m d ) Fig. 10. POC export flux plotted against benthic C respiration indicating a tight coupling between organic C export and benthic respiration. Circled data points are from stations with water depth 4400 m.

benthos in the productive summer months, resulting in a seasonal decoupling between POC export and benthic respiration. The organic C balance for this shelf-slope regime in the spring and summer is evaluated by comparing measured rates of primary productivity, POC export flux, and benthic C respiration (Table 3, Fig. 11). In the spring, 19% of shelf and 15% of slope production was exported from the upper waters and entirely respired by benthic organisms (Table 3, Fig. 11). Note that the very high POC

export flux measured at Station 37-BC3 was omitted from this comparison; this latter station was located in Barrow Canyon, which has a rocky bottom and high lateral current flows. The spring results are therefore consistent with a tight coupling between primary productivity, POC export, and benthic respiration that minimizes the off-shelf export of POC. This is consistent with previous studies conducted in the Chukchi shelf and Chirikov Basin (northern Bering Sea), which indicate 10–50% of exported POC was respired in the sediments (Grebmeier, 1987; Grebmeier and McRoy, 1989). The tight coupling of POC export fluxes and benthic carbon respiration rates in spring is partly due to the fact that the available organic carbon more likely reaches the benthos intact in spring when cold temperatures limit microbial cycling (Kirchman et al., 2005) and zooplankton growth (Plourde et al., 2005), and thus grazing pressure is reduced. In contrast, during the productive summer months, 37% of shelf and 28% of slope production, respectively, were exported from the upper waters (Table 3, Fig. 11). Importantly, only 20% of shelf and 9% of slope production were simultaneously respired by benthic animals and microbes. The apparent decoupling of export production and benthic carbon respiration in the summer may reflect either the time lag for POC exported from the upper waters to reach the sediment/water interface or a change in the efficiency of benthic respiration. For both the shelf and slope in the summer, approximately 18% (17–19%) of export production remained unconsumed through benthic respiration. The implication is that up to 20%, or 6
7 mmol C m2 d1 , of total shelf-slope production in the summer may be decoupled from benthic respiration and seasonally exported to the deeper slope waters and sediments. Furthermore, if POC export fluxes are underestimated during the summer when production is increasing, due to our use of a 1D steady-state model (Eq. (1)), then the seasonal decoupling between POC export and benthic carbon mineralization, and hence off-shelf carbon export, would be even larger. As noted above, the benthic O2 uptake data represent minimum values for sediment carbon mineralization. However, assuming an 18–39% higher benthic carbon mineralization rate, due to the inclusion of anaerobic respiration rates previously reported for western Arctic shelf sediments (Devol et al., 1997) and 2002 SBI sampled sediments (generally o1–5 mmol m2 d1 ; A. Devol, pers.

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(A)

3443

(B)

Spring

Summer Primary productivity: 12

Shelf POC export: 2.8

Slope POC export: 1.3

Primary productivity: 33 Shelf POC export: 13

Slope POC export: 8.8

∆ POC: – 0.5 Shelf benthic C respiration: 3.3

Slope benthic C respiration: 1.5

∆ POC: 5.8 Shelf benthic C respiration: 7.2

Slope benthic C respiration: 2.7

Fig. 11. Seasonal change in organic carbon fluxes ðmmol C m2 d1 Þ during the SBI process study in the Chukchi Sea, 2002. Primary productivity is total ðshelf þ slopeÞ. DPOC ¼ total ðshelf þ slopeÞ POC export  total benthic C respiration. Note that shelf stations are defined as o300 m.

comm.), the summer benthic respiration rates remain decoupled from the higher POC export fluxes at these stations. Thus, we conclude that seasonal POC export to the deeper slope waters and sediments, maximized in the summer, is occurring at the shelf-break region. This conclusion holds even assuming an average 25% increase in carbon mineralization due to anaerobic respiration in spring and summer on both the shelf and slope. 4.6. DOC accumulation A complete budget of C export must include the net autochthonous production (accumulation) of DOC in the system. Unfortunately, net DOC production was not directly measurable because of the confounding influences of ice melt and river input on mixed layer DOC concentrations (Mathis et al., 2005). We can, however, make indirect estimates of the net production of DOC because DOC accumulates as a small percentage ( 15%) of net community production (NCP) in eutrophic environments (Hansell et al., 1997a,b; Hansell and Carlson, 1998). An estimate for DOC accumulation can be made after first calculating particulate NCP at each station. NCP was calculated as the sum of the integrated accumulation of suspended POC in the upper 50 m (Bates et al., 2005) and the flux of POC exported from this layer estimated using 234 Th (Table 1). The net accumulation of suspended POC was calculated by subtracting the mean integrated

background POC concentration in the upper 50 m in the spring ð92 mmol C m2 Þ from the integrated POC concentration at each station in the summer. The rate of accumulation of POC for the summer was then calculated by dividing the total integrated POC concentration by the 60-day growth period between these cruises (Table 3). Net DOC production was taken as 15% of this daily rate. Based on these calculations, DOC accumulation associated with net community production represents 6% of primary production, or 1:9
1:3 mmol C m2 d1 in the shelf and slope waters in the summer (Table 3). While there is considerable uncertainty associated with the indirect method required for these calculations, the implication is that there is a relatively minor off-shelf export of marine produced DOC compared to POC export on a seasonal basis in the Chukchi Sea. Finally, we have not considered here the contribution of terrigenous DOC introduced by rivers. The Mackenzie and Yukon Rivers, the major local rivers, deliver 1.3 and 0:9  1012 g C, respectively, as DOC into the system annually (Cauwet, 2002). It is not known to what extent the in-flux of riverine DOC may be exported off-shelf into the interior basin. 5. Conclusions Seasonal measurements of 234 Th/238 U disequilibrium in the Chukchi Sea demonstrate the importance of short-term changes in the flux of

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3444

particles and associated organic C exported from the euphotic zone and of the potential for exchange of POC between the shelf and interior slope/basin. These results provide some of the first direct evidence of the strong seasonal coupling between export production and benthic respiration in the shelf/slope region of the Arctic Ocean. Enhanced scavenging and particle export was evident in the shelf and slope waters, with the highest particle export focused within Barrow Canyon. A marked seasonal increase in particle export was clearly observed at all stations during the transition from spring to summer. The seasonal change in dissolved-particulate partitioning of 234 Th is consistent with the net removal of dissolved 234 Th by particle scavenging in response to the increase in primary and export production. In the sub-surface waters, below the euphotic zone, low total 234 Th/ 238 U activity ratios are attributed to enhanced scavenging of 234 Th from shelf water and subsequent off-shelf transport into the interior slope waters on a time-scale of several weeks. 234 Th-derived POC fluxes exhibit significant seasonal and spatial variability, increasing approximately 4-fold in response to a similar increase in primary productivity. The fraction of primary production exported from the upper shelf/slope waters increases from 15% in the spring to 32% in the summer. By comparison, estimates of DOC accumulation associated with net community production represented 6% of primary production (2 mmol C m2 d1 ). The majority of shelf and

slope stations indicate a good agreement between POC export and benthic C respiration in the spring, and hence a tight coupling between primary productivity, POC export, and benthic respiration. In the summer, there is an imbalance between POC export and benthic respiration, implying that up to 20% of summer production ð6
7 mmol C m2 d1 Þ may be seasonally exported offshelf.

Acknowledgements We thank the Captain, Officers, Marine Science Technicians, and crew of the U.S.C.G.C. Healy for their assistance with field sampling. Richard Kingsley assisted with the ICPMS analysis. We wish to acknowledge the efforts of our friend and colleague, the late Glenn Cota. Constructive comments of two anonymous reviewers improved the paper. This work was supported by the NSF (OPP-0124917 to SBM; OPP-0125082 to JMG and LWC; OPP0125049 to GFC; OPP-0124864 to JJW; OPP0124868 to NRB; OPP-0124900 to DAH; OPP0124943 to WM).

Appendix A 234

Th, 238U, POC, and PON data from the Chukchi Sea, 2002; SBI-II process cruises HLY02-01 and HLY-02-03 are provided in Table A1.

Table A1 234

234

238

(dpm l )

(dpm l )

(dpm l )

Spring, May 6– June 15, 2002 6-WHS2, 72.901 N, 160.504 W, 60 m 5 0:75
0:08 0:32
0:02 20 0:10
0:10 0:02
0:01 40 0:42
0:09 0:39
0:03 55 0:28
0:09 0:28
0:02

0:014
0:0010 0:001
0:0001 0:034
0:0024 0:034
0:0024

1:09
0:08 0:12
0:10 0:84
0:10 0:59
0:09

2.15 2.16 2.20 2.26

0:51
0:04 0:06
0:05 0:38
0:04 0:26
0:04

0.203 0.031 0.105 0.124

0.034 0.003 0.015 0.017

5.9 11.7 6.9 7.4

7-WHS3, 5 20 40 75 100

73.037 N, 160.428 W, 176 m 0:97
0:12 0:14
0:02 0:22
0:04 0:35
0:03 0:85
0:10 0:76
0:05 0:10
0:10 0:02
0:01 1:25
0:18 0:16
0:02

0:013
0:0009 0:067
0:0047 0:032
0:0023 0:013
0:0009 0:035
0:0024

1:12
0:12 0:64
0:05 1:64
0:11 0:13
0:10 1:45
0:18

2.12 2.13 2.16 2.25 2.25

0:53
0:06 0:30
0:02 0:76
0:05 0:06
0:04 0:64
0:08

0.129 0.273 0.077 0.061 0.155

0.018 0.029 0.009 0.007 0.016

7.2 9.4 8.3 9.4 9.8

8-WHS4, 5 35 75

73.260 N, 160.095 W, 889 m 2:28
0:16 0:04
0:01 0:004
0:0003 1:53
0:14 0:05
0:01 0:031
0:0022 1:32
0:17 0:40
0:04 0:023
0:0016

2:32
0:16 1:60
0:14 1:74
0:18

2.07 2.12 2.24

1:12
0:08 0:76
0:07 0:78
0:08

0.098 0.077 0.131

0.014 0.008 0.015

6.8 9.8 8.8

(m)

234

234

234 ThTotal =238 U (AR)

Depth

Thdiss 1

(dpm l )

Thpart 1

(dpm l )

Th453 mm 1

ThTotal 1

U 1

POC453 mm 1

(mmol l )

PON453 mm

C=N453 mm

1

(mmol l )

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3445

Table A1 (continued ) 234

234

234

234

238

234

(dpm l )

(dpm l )

(dpm l )

ThTotal =238 U (AR)

0:044
0:0031 0:035
0:0025

1:67
0:10 2:39
0:34

2.28 2.40

0:73
0:04 0:99
0:14

0.033 0.023

0.004 0.003

8.9 7.3

73.356 N, 160.380 W, 1190 m 2:02
0:20 0:12
0:01 0:002
0:0001 1:56
0:19 0:46
0:03 0:052
0:0036 1:70
0:23 0:40
0:04 0:030
0:0021 1:92
0:24 0:16
0:02 0:018
0:0012 2:05
0:27 0:23
0:02 0:010
0:0007

2:14
0:20 2:06
0:19 2:13
0:24 2:09
0:24 2:28
0:28

2.08 2.20 2.20 2.40 2.40

1:03
0:10 0:94
0:09 0:97
0:11 0:87
0:10 0:95
0:11

0.161 0.031 0.021 0.012 0.009

0.026 0.003 0.003 0.002 0.002

6.3 9.0 6.7 6.7 5.4

10-WHS6, 73.444 N, 159.795 W, 1872 m 5 1:68
0:16 0:05
0:01 0:007
0:0005 50 2:07
0:23 0:07
0:01 0:010
0:0007 100 1:73
0:22 0:21
0:04 0:022
0:0015 300 2:36
0:29 0:11
0:02 0:006
0:0004 600 2:24
0:27 0:1
0:02 0:008
0:0006

1:74
0:16 2:15
0:23 1:97
0:22 2:47
0:29 2:35
0:28

2.04 2.19 2.25 2.40 2.41

0:85
0:08 0:98
0:10 0:87
0:10 1:03
0:12 0:97
0:11

0.242 0.014 0.005 0.003 0.001

0.039 0.002 0.001 ND 0.001

6.2 8.2 5.2 ND 0.6

11-WHS7, 73.776 N, 159.051 W, 3071 m 5 1:73
0:18 0:05
0:01 0:008
0:0005 50 1:13
0:15 0:05
0:01 0:010
0:0007 100 1:68
0:21 0:32
0:03 0:013
0:0009 300 2:28
0:19 0:11
0:01 0:012
0:0009 600 2:40
0:36 0:09
0:03 0:005
0:0004

1:79
0:18 1:19
0:15 2:02
0:21 2:40
0:19 2:50
0:36

2.02 2.18 2.25 2.40 2.40

0:88
0:09 0:54
0:07 0:90
0:09 1:00
0:08 1:04
0:15

0.189 0.018 0.001 0.001 0.006

0.022 0.002 0.001 0.001 ND

8.5 8.3 0.6 1.0 ND

12-EHS11, 73.438 N, 157.535 W, 2878 m 5 1:79
0:21 0:07
0:02 0:023
0:0016 20 1:78
0:24 0:02
0:02 0:017
0:0012 75 1:92
0:20 0:19
0:04 0:015
0:0011 125 1:42
0:17 0:51
0:03 0:017
0:0012 300 2:22
0:36 0:19
0:03 0:006
0:0005

1:88
0:21 1:83
0:24 2:13
0:21 1:95
0:18 2:42
0:36

2.05 2.06 2.23 2.27 2.40

0:92
0:10 0:89
0:12 0:95
0:09 0:86
0:08 1:01
0:15

0.279 0.142 0.001 0.001 0.001

0.035 0.020 ND ND ND

7.9 7.0 ND ND ND

14-EHS9, 73.091N, 158.182 W, 2108 m 5 1:53
0:20 0:1
0:02 0:020
0:0014 20 1:48
0:14 0:13
0:02 0:056
0:0039 75 1:81
0:23 0:36
0:03 0:018
0:0012 125 1:74
0:26 0:06
0:01 0:017
0:0012 300 2:19
0:46 0:22
0:04 0:033
0:0023

1:66
0:21 1:67
0:14 2:19
0:23 1:82
0:26 2:44
0:46

2.10 2.10 2.23 2.28 2.40

0:79
0:10 0:80
0:07 0:98
0:11 0:80
0:11 1:02
0:19

0.246 0.339 0.003 0.001 0.006

0.039 0.049 0.001 0.001 0.001

6.3 7.0 4.0 1.8 3.9

16-EHS7, 72.873 N, 158.361 W, 970 m 5 1:35
0:17 0:23
0:02 20 1:62
0:22 0:22
0:04 75 1:24
0:18 0:64
0:08 125 1:12
0:19 0:72
0:09 300 2:40
0:31 0:2
0:03

0:050
0:0035 0:026
0:0018 0:013
0:0009 0:040
0:0028 0:073
0:0051

1:63
0:17 1:87
0:23 1:89
0:20 1:86
0:21 2:67
0:31

2.11 2.12 2.22 2.27 2.40

0:77
0:08 0:88
0:11 0:85
0:09 0:82
0:09 1:11
0:13

0.343 0.138 0.005 0.010 0.077

0.048 0.020 0.001 0.002 0.012

7.2 6.9 4.7 5.2 6.7

18-EHS5, 72.39 N, 158.631 W, 236 m 5 1:78
0:19 0:08
0:02 20 1:86
0:26 0:03
0:04 40 1:94
0:21 0:03
0:02 80 1:06
0:15 0:64
0:03 120 0:15
0:08 0:18
0:02

0:013
0:0009 0:052
0:0037 0:059
0:0041 0:059
0:0041 0:018
0:0012

1:87
0:19 1:95
0:26 2:03
0:22 1:75
0:15 0:35
0:08

2.11 2.11 2.12 2.21 2.27

0:89
0:09 0:92
0:12 0:96
0:10 0:79
0:07 0:15
0:04

0.215 0.430 0.345 0.039 0.010

0.034 0.053 0.060 0.005 0.001

6.3 8.1 5.8 8.1 10.8

19-EHS4, 72.603 N, 158.940 W, 83 m 7 1:48
0:18 0:18
0:02 15 1:42
0:28 0:18
0:03 35 1:24
0:24 0:22
0:03 65 0:65
0:23 0:7
0:08

0:017
0:0012 0:063
0:0044 0:075
0:0053 0:042
0:0030

1:67
0:18 1:66
0:28 1:53
0:25 1:39
0:24

2.12 2.13 2.14 2.23

0:79
0:09 0:78
0:13 0:71
0:11 0:62
0:11

0.157 0.224 0.259 0.072

0.022 0.025 0.031 0.010

7.0 9.0 8.4 7.3

24-BC2A, 71.821 N, 155.744 W, 100 m 5 1:44
0:17 0:09
0:04 0:042
0:0030 20 1:96
0:29 0:22
0:04 0:035
0:0025 40 1:10
0:16 0:6
0:04 0:078
0:0023 80 0:56
0:49 0:6
0:06 0:035
0:0025

1:58
0:18 2:22
0:29 1:77
0:17 1:20
0:50

2.13 2.15 2.17 2.25

0:74
0:08 1:03
0:13 0:82
0:08 0:53
0:22

0.168 0.101 0.051 0.059

0.016 0.011 0.006 0.006

10.5 9.4 8.1 9.3

Depth

Thdiss 1

(m)

(dpm l )

120 300

1:12
0:09 2:15
0:34

9-WHS5, 5 50 120 300 500

Thpart 1

(dpm l ) 0:51
0:03 0:20
0:02

Th453 mm 1

ThTotal 1

U 1

POC453 mm

PON453 mm

(mmol l1 )

(mmol l1 )

C=N453 mm

ARTICLE IN PRESS S.B. Moran et al. / Deep-Sea Research II 52 (2005) 3427–3451

3446 Table A1 (continued ) Depth (m)

234

Thdiss 1

(dpm l )

234

Thpart 1

(dpm l )

234

Th453 mm 1

(dpm l )

234

238

234

(dpm l )

(dpm l )

ThTotal =238 U (AR)

ThTotal 1

U 1

POC453 mm

PON453 mm

(mmol l1 )

(mmol l1 )

C=N453 mm

31-BC4, 7 20 75 125 300

71.928 N, 154.847 W, 548 m 1:33
0:16 0:51
0:04 0:022
0:0016 1:16
0:21 0:58
0:06 0:075
0:0052 1:05
0:16 0:36
0:07 0:030
0:0021 1:15
0:17 0:63
0:04 0:028
0:0020 1:11
0:24 0:23
0:05 0:045
0:0032

1:86
0:16 1:81
0:22 1:44
0:17 1:81
0:18 1:38
0:24

2.15 2.15 2.23 2.31 2.40

0:86
0:08 0:84
0:10 0:64
0:08 0:78
0:08 0:58
0:10

0.211 1.940 0.035 0.019 0.077

0.034 0.023 0.004 0.002 0.001

6.3 83.6 8.6 8.7 151.2

32-BC5, 7 20 75 125 300

72.109 N, 154.492 W, 1551 m 1:05
0:13 0:37
0:04 1:36
0:17 0:4
0:06 0:76
0:14 0:22
0:04 1:00
0:15 0:68
0:06 0:57
0:20 0:51
0:09

0:017
0:0012 0:068
0:0048 0:029
0:0020 0:025
0:0017 0:052
0:0036

1:44
0:14 1:83
0:18 1:01
0:14 1:71
0:17 1:13
0:22

2.15 2.15 2.22 2.30 2.40

0:67
0:06 0:85
0:08 0:45
0:07 0:74
0:07 0:47
0:09

0.488 0.077 0.024 0.021 0.115

0.098 0.010 0.003 0.002 0.011

5.0 7.7 7.5 8.2 10.4

33-BC6, 7 20 75 125 300

72.182 N, 154.270 W, 1849 m 1:78
0:30 0:25
0:05 1:60
0:23 0:29
0:05 1:18
0:15 0:52
0:08 0:83
0:18 0:37
0:05 0:74
0:18 0:35
0:06

0:034
0:0024 0:087
0:0061 0:031
0:0022 0:036
0:0025 0:021
0:0015

2:06
0:31 1:98
0:23 1:73
0:17 1:24
0:18 1:11
0:19

2.14 2.18 2.24 2.29 2.39

0:96
0:14 0:91
0:11 0:77
0:08 0:54
0:08 0:46
0:08

0.472 0.298 0.018 0.051 0.033

0.059 0.038 0.003 0.006 0.003

8.0 7.9 7.0 8.5 12.4

34-BC7, 7 20 75 125 300

72.561 N, 154.613 W, 2933 m 1:91
0:20 0:08
0:03 2:04
0:34 0:03
0:07 1:61
0:33 0:44
0:06 1:25
0:26 1:19
0:19 1:89
0:35 0:26
0:11

0:016
0:0011 0:017
0:0012 0:010
0:0007 0:021
0:0015 0:039
0:0028

2:01
0:21 2:09
0:35 2:06
0:34 2:46
0:32 2:19
0:36

2.06 2.06 2.24 2.28 2.4

0:97
0:10 1:01
0:17 0:92
0:15 1:08
0:14 0:91
0:15

0.245 0.173 0.006 0.012 0.021

0.024 0.017 0.001 0.001 0.002

10.3 10.4 11.4 11.0 13.2

37-BC3, 7 20 75 100 140

71.649 N, 155.763 W, 190 m 1:07
0:30 0:58
0:12 1:07
0:33 0:28
0:16 1:11
0:32 0:35
0:15 0:90
0:32 0:48
0:17 0:53
0:30 0:87
0:21

0:087
0:0061 0:234
0:0164 0:192
0:0134 0:011
0:0008 0:025
0:0017

1:74
0:33 1:58
0:36 1:66
0:35 1:39
0:36 1:43
0:36

2.17 2.2 2.23 2.25 2.31

0:80
0:15 0:72
0:16 0:74
0:16 0:62
0:16 0:62
0:16

3.057 6.434 6.295 0.042 0.330

0.604 1.287 1.195 0.005 0.046

5.1 5.0 5.3 8.0 7.2

Summer, July 17-August 26, 2002 11-BC2, 71.392 N, 157.497 W, 118 m 7 0:50
0:05 0:18
0:03 0:174
0:0122 20 0:50
0:10 0:24
0:05 0:374
0:0262 75 0:25
0:07 0:66
0:10 0:146
0:0102 100 0:29
0:08 0:21
0:03 0:109
0:0076

0:86
0:06 1:12
0:11 1:06
0:12 0:61
0:09

2.15 2.19 2.27 2.28

0:40
0:03 0:51
0:05 0:46
0:05 0:27
0:04

3.173 7.647 2.479 1.593

0.001 0.270 0.184 0.075

3588 28.31 13.44 21.27

13-BC3, 7 20 40 75 100

71.622 N, 155.9732 W, 182 m 0:59
0:08 0:10
0:01 0:41
0:11 0:24
0:05 0:63
0:15 0:26
0:09 0:16
0:06 0:37
0:05 0:18
0:04 0:35
0:04

0:123
0:0086 0:128
0:0090 0:422
0:0295 0:064
0:0045 0:047
0:0033

0:81
0:08 0:78
0:11 1:31
0:18 0:60
0:07 0:58
0:05

2.05 2.13 2.18 2.25 2.29

0:40
0:04 0:37
0:05 0:60
0:08 0:27
0:03 0:25
0:02

3.734 5.238 6.241 0.534 0.556

0.138 0.467 0.343 0.064 0.072

27.03 11.23 18.20 8.34 7.67

14-BC4, 7 25 75 125 300

71.923 N, 154.886 W, 601 m 0:48
0:08 0:23
0:03 0:106
0:0074 0:81
0:18 0:20
0:03 0:143
0:0100 0:82
0:19 0:65
0:10 0:062
0:0044 0:27
0:06 0:65
0:09 0:049
0:0034 0:89
0:12 0:20
0:04 0:065
0:0045

0:82
0:08 1:16
0:18 1:54
0:22 0:96
0:11 1:16
0:12

2.02 2.17 2.26 2.31 2.40

0:41
0:04 0:53
0:08 0:68
0:10 0:42
0:05 0:48
0:05

2.166 2.122 0.188 0.280 0.076

0.115 0.091 0.021 0.038 0.010

18.81 23.24 9.13 7.40 7.88

16-BC6, 7 25 75 125 300 600 1500

72.235 N, 153.903 W, 2235 m 1:13
0:12 0:41
0:07 0:50
0:14 0:40
0:09 0:42
0:09 0:17
0:07 0:77
0:13 0:42
0:06 0:71
0:22 0:19
0:04 2:07
0:23 0:11
0:04 2:42
0:27 0:07
0:04

1:57
0:14 1:09
0:16 0:83
0:12 1:22
0:14 0:90
0:23 2:19
0:23 2:50
0:27

1.94 2.09 2.25 2.28 2.40 2.41 2.41

0:81
0:07 0:52
0:08 0:37
0:05 0:53
0:06 0:38
0:10 0:91
0:10 1:04
0:11

1.135 3.881 2.741 0.916 0.004 0.006 0.021

0.017 0.356 0.270 0.012 0.001 0.001 0.001

64.94 10.91 10.14 77.30 7.05 7.99 16.81

0:025
0:0017 0:186
0:0130 0:249
0:0174 0:031
0:0022 0:009
0:0006 0:007
0:0005 0:019
0:0013

ARTICLE IN PRESS S.B. Moran et al. / Deep-Sea Research II 52 (2005) 3427–3451

3447

Table A1 (continued ) Depth (m)

234

Thdiss 1

(dpm l )

234

Thpart 1

(dpm l )

234

234

238

234

(dpm l )

(dpm l )

(dpm l )

ThTotal =238 U (AR)

Th453 mm 1

ThTotal 1

U 1

POC453 mm

PON453 mm

(mmol l1 )

(mmol l1 )

C=N453 mm

19-EB6, 7 25 75 125 300 600 1500

71.945 N, 152.075 W, 2193 m 0:46
0:10 0:17
0:03 0:82
0:19 0:25
0:13 0:55
0:14 0:42
0:08 0:83
0:15 0:49
0:07 0:46
0:12 1:08
0:26 1:31
0:21 0:03
0:12 2:03
0:25 0:25
0:07

0:118
0:0083 0:247
0:0173 0:175
0:0122 0:069
0:0048 0:027
0:0019 0:013
0:0009 0:042
0:0030

0:75
0:10 1:31
0:22 1:14
0:16 1:39
0:16 1:57
0:29 1:35
0:24 2:32
0:26

1.95 2.13 2.27 2.30 2.40 2.41 2.41

0:38
0:05 0:62
0:11 0:51
0:07 0:60
0:07 0:65
0:12 0:56
0:10 0:96
0:11

3.539 4.360 0.687 0.213 0.014 0.008 0.028

0.200 0.315 0.046 0.026 0.002 0.002 0.003

17.72 13.84 14.85 8.18 7.79 3.75 8.98

20-EB5, 7 25 75 125 300

71.676 N, 152.105 W, 2306 m 0:80
0:09 0:26
0:04 1:02
0:17 0:19
0:08 0:37
0:12 0:43
0:09 0:48
0:07 0:33
0:05 0:91
0:06 0:46
0:04

0:018
0:0013 0:181
0:0127 0:025
0:0017 0:018
0:0013 0:045
0:0032

1:08
0:10 1:39
0:19 0:83
0:15 0:83
0:09 1:42
0:07

1.93 2.12 2.25 2.30 2.40

0:56
0:05 0:66
0:09 0:37
0:07 0:36
0:04 0:59
0:03

0.376 2.357 0.136 0.042 0.045

0.046 0.109 0.019 0.006 0.006

8.20 21.63 7.20 7.11 7.93

21-EB4, 7 25 75

71.642 N, 152.367 W, 406 m 1:01
0:17 0:25
0:04 0:91
0:18 0:33
0:10 0:47
0:18 0:47
0:08

0:048
0:0034 0:098
0:0069 0:130
0:0091

1:30
0:13 1:34
0:21 1:06
0:19

1.93 2.11 2.25

0:67
0:07 0:63
0:10 0:47
0:08

0.884 0.881 1.468

0.127 0.112 0.207

6.95 7.88 7.08

22-EB3, 7 20 40 75 100

71.568 N, 152.331 W, 197 m 0:37
0:08 0:17
0:04 0:49
0:09 0:27
0:06 0:90
0:44 0:22
0:14 0.48
0:19 0:39
0:09 0:76
0:19 0:74
0:13

0:002
0:0002 0:034
0:0023 0:142
0:0099 0:212
0:0149 0:106
0:0074

0:54
0:09 0:80
0:11 1:27
0:46 1:08
0:33 1:61
0:23

1.84 2.08 2.19 2.23 2.24

0:30
0:05 0:38
0:05 0:58
0:21 0:49
0:15 0:72
0:10

0.240 0.323 1.140 1.073 0.691

0.049 0.016 0.143 0.123 0.096

4.906 19.669 7.965 8.756 7.187

23-EB2, 7 20 40 60 75

71.449 N, 152.550 W, 85 m 0:32
0:06 0:26
0:05 1:02
0:25 0:15
0:08 0:94
0:16 0:20
0:10 0:66
0:26 0:42
0:15 0:64
0:20 0:01
0:23

0:004
0:0003 0:054
0:0038 0:057
0:0040 0:100
0:0070 0:095
0:0066

0:58
0:08 1:22
0:27 1:19
0:19 1:18
0:30 0:75
0:30

1.81 2.04 2.17 2.20 2.21

0:32
0:04 0:60
0:13 0:55
0:09 0:53
0:14 0:34
0:14

0.425 0.867 0.490 1.976 2.057

0.067 0.072 0.028 0.187 0.212

6.33 12.04 17.24 10.57 9.69

26-EHS4, 72.648 N, 158.616 W, 158 m 7 0:85
0:14 0:29
0:07 0:025
0:0018 40 3:75
0:63 0:08
0:20 0:124
0:0087 75 0:60
0:23 0:56
0:12 0:049
0:0034

1:17
0:16 3:95
0:66 1:21
0:26

1.97 2.19 2.26

0:59
0:08 1:80
0:30 0:53
0:11

0.330 0.522 0.216

0.045 0.066 0.029

7.33 7.89 7.46

27-EHS5, 72.690 N, 158.586 W, 245 m 7 0:94
0:11 0:21
0:05 20 1:07
0:45 0:34
0:12 40 0:87
0:92 0:01
0:10 75 1:55
0:25 0:86
0:16

0:023
0:0016 0:092
0:0064 0:066
0:0046 0:077
0:0054

1:17
0:12 1:51
0:46 0:95
0:93 2:49
0:30

2.58 2.07 2.19 2.25

0:45
0:05 0:73
0:22 0:43
0:42 1:11
0:13

0.210 0.667 0.117 0.067

0.026 0.096 0.017 0.009

8.00 6.93 6.93 7.65

28-EHS6, 72.825 N, 159.330 W, 403 m 7 0:73
0:24 0:15
0:08 25 1:04
0:73 0:13
0:11 75 0:74
0:53 0:04
0:16 125 0:78
0:97 0:17
0:15

0:006
0:0004 0:154
0:0108 0:049
0:0034 0:034
0:0024

0:89
0:25 1:32
0:34 0:83
0:35 0:98
0:38

1.88 2.10 2.24 2.28

0:47
0:13 0:63
0:16 0:37
0:16 0:43
0:17

0.088 1.169 0.042 0.031

0.015 0.169 0.006 0.006

5.91 6.94 6.50 5.30

29-EHS7, 72.888 N, 158.362 W, 978 m 7 0:63
0:21 0:10
0:03 25 1:09
0:45 0:14
0:20 75 1:41
0:75 0:18
0:12 125 0:80
0:45 0:70
0:30 300 0:99
0:50 0:11
0:24 500 0:96
0:29 0:12
0:15

0:006
0:0004 0:084
0:0058 0:033
0:0023 0:045
0:0031 0:028
0:0019 0:056
0:0039

0:74
0:21 1:23
0:33 1:62
0:76 1:55
0:38 1:13
0:34 1:08
0:32

1.93 2.10 2.24 2.28 2.40 2.40

0:38
0:11 0:59
0:16 0:72
0:34 0:68
0:17 0:47
0:14 0:45
0:13

0.196 0.623 0.050 0.042 0.017 0.052

0.026 0.104 0.008 0.005 0.003 0.006

7.68 5.99 6.01 7.79 5.49 8.08

30-EHS9, 73.063 N, 157.917 W, 2024 m 7 0:92
0:07 0:07
0:05 0:007
0:0005 75 1:18
0:75 0:11
0:15 0:028
0:0019

0:99
0:08 1:29
0:24

1.95 2.23

0:51
0:04 0:58
0:11

0.194 0.033

0.021 0.006

9.31 5.36

ARTICLE IN PRESS S.B. Moran et al. / Deep-Sea Research II 52 (2005) 3427–3451

3448 Table A1 (continued ) 234

234

234

234

238

234

(dpm l )

(dpm l )

(dpm l )

ThTotal =238 U (AR)

0:033
0:0023 0:021
0:0015 0:016
0:0011 0:006
0:0004

0:89
0:44 1:79
0:69 1:94
0:34 2:16
0:26

2.27 2.40 2.40 2.41

0:39
0:19 0:75
0:29 0:81
0:14 0:90
0:11

0.013 0.002 0.004 0.001

0.002 ND 0.001 0.001

5.48 ND 3.60 2.03

33-WHS6, 73.500 N, 159.474 W, 2264 m 7 1:32
0:07 0:29
0:03 0:002
0:0001 25 1:60
0:09 0:34
0:03 0:044
0:0031 75 1:29
0:07 0:30
0:03 0:043
0:0030 125 1:00
0:27 0:21
0:02 0:022
0:0015 300 1:23
0:26 0:06
0:01 0:009
0:0006 600 1:07
0:11 0:05
0:01 0:008
0:0006 1500 2:17
0:25 0:11
0:01 0:006
0:0004

1:61
0:07 1:98
0:10 1:60
0:08 1:24
0:27 1:30
0:26 1:14
0:11 2:28
0:25

1.87 2.04 2.23 2.27 2.40 2.40 2.41

0:86
0:04 0:97
0:05 0:72
0:03 0:54
0:12 0:54
0:11 0:47
0:05 0:95
0:10

0.037 0.228 0.073 0.004 0.001 0.004 0.002

0.006 0.033 0.009 0.001 0.002 0.001 ND

5.865 6.972 8.203 4.013 0.342 3.325 ND

34-WHS5, 73.295 N, 160.238 W, 1108 m 7 1:21
0:03 0:21
0:02 0:022
0:0016 25 1:14
0:07 0:22
0:02 0:169
0:0118 75 0:84
0:30 0:18
0:02 0:083
0:0058 125 0:90
0:36 0:07
0:01 0:018
0:0013 300 1:67
0:21 0:08
0:01 0:009
0:0007

1:49
0:04 1:55
0:07 1:10
0:30 1:11
0:36 1:77
0:21

1.89 2.11 2.25 2.30 2.40

0:79
0:02 0:74
0:03 0:49
0:13 0:48
0:16 0:75
0:09

0.192 0.976 0.095 0.012 0.007

0.013 0.124 0.013 0.002 0.001

14.318 7.876 7.174 6.007 5.209

35-WHS4, 73.225 N, 160.133 W, 485 m 7 0:84
0:04 0:18
0:02 0:157
0:0110 25 1:19
0:09 0:18
0:02 0:119
0:0083 75 0:76
0:03 0:16
0:02 0:057
0:0040 300 1:68
0:28 0:08
0:01 0:028
0:0020

1:08
0:05 1:56
0:09 0:98
0:04 1:79
0:28

1.90 2.15 2.27 2.39

0:57
0:02 0:72
0:04 0:43
0:02 0:75
0:12

1.009 0.771 0.078 0.017

0.027 0.105 0.011 0.004

36.815 7.366 7.176 3.718

38-WHS2, 73.011 N, 160.646 W, 136 m 20 1:00
0:16 0:21
0:02 0:350
0:0245 40 1:07
0:07 0:23
0:02 0:130
0:0091 60 0:97
0:13 0:20
0:02 0:023
0:0016 75 0:16
0:03 0:03
0:01 0:043
0:0030

1:55
0:16 1:43
0:07 1:20
0:13 0:24
0:03

2.15 2.24 2.28 2.28

0:72
0:08 0:64
0:03 0:53
0:06 0:10
0:01

2.455 0.577 0.168 0.270

0.146 0.047 0.026 0.028

16.866 12.271 6.546 9.536

Depth

Thdiss 1

(m)

(dpm l )

125 300 600 1500

0:85
0:60 1:69
0:58 1:86
0:32 2:09
0:15

Thpart 1

(dpm l ) 0:01
0:09 0:08
0:38 0:06
0:11 0:07
0:22

Th453 mm 1

ThTotal 1

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