Sources, Fluxes, and Accumulation and Burial Rates of Organic Carbon in Continental Margin

7.2 The continental margin of the North Bering-Chukchi Sea: concentrations, sources, fluxes, accumulation and burial rates of organic carbon

A. S. Naidu1*, L. W. Cooper2, J. M. Grebmeier2, T. E. Whitledge1 and M. J. Hameedi3 1 Institute of Marine Science, University of Alaska, Fairbanks, AK 99775, USA 2 Department of Ecology and Evolutionary Biology, 10515 Research Drive, suite 100, Bldg. A, University of Tennessee, Knoxville, TN 37932, USA 3 NOAA, SC1, 1305 East-West Highway, Silverspring, MD 20910

* Corresponding Author, E-mail: [email protected]; Fax: +1-907-474-7204

7.2.1 Introduction

The seasonally ice-covered contiguous continental margins of Bering and Chukchi Seas are some of the largest margins in the world; the U.S. portion comprises more than 50% of the total Exclusive Economic Zone of the United States. These margins have several unique environmental features that set them apart from other high Arctic margins. The region is bounded in the east and west by continents, as opposed to the rest of the arctic margins, which have land to the south and open ocean to the north. Additionally, there are several sharp cross and along shelf gradients in environmental parameters. For example, nutrient-rich basin waters upwelling along the continental slope of the Bering Sea and advecting onto the adjacent northwest Bering shelf successively give way along their migration northward to nutrient-poor waters. Across the North Bering-Chukchi margin there are also broad east to west gradients in nutrient concentration, temperature, current and salinity. A factor that contributes to the latter gradients is the presence of relatively larger, nutrient-poor fluvial flux in the east and the advection of nutrient-rich basin waters in the west. As is to be expected, the primary production rate parallels the nutrient concentrations in overlying waters and has, within the nutrient-enriched plume the highest rate (up to 720-840 g C m-2 y-1; Springer and McRoy, 1993) among all the adjacent arctic margins. The gradients have combined to develop a distinct north-south aligned hydrographic front that bisects the study area into two broad water masses (Walsh et al., 1989, and references therein).

There are additional characteristics of the North Bering-Chukchi Sea that are different from rest of the arctic margins. The recent paleogeographic history of the area has been quite distinct, inasmuch as most of the region during the late Pleistocene was not glaciated and, instead, was a fluvial-dominated coastal plain. This is reflected in the present sea bottom topography, which determines the present pathways of current flows (Fig.1), and also extensively in the nature of surficial sediments, which influence the distribution of benthic community there. Further, the Bering-Chukchi Sea region has low degree of seasonal sea ice cover relative to other Arctic shelves and the longest migration of the ice edge between seasons. All these lateral and latitudinal environmental gradients no doubt influence the regional budget of organic carbon. It is likely that the budget of

1 organic carbon in the very productive Northern Bering-Southern Chukchi Sea differs from the other arctic shelves which are relatively less productive. The major goal of this chapter is to synthesize published and unpublished data on the concentrations, sources, net fluxes, and rates of accumulation, burial, remineralization of particulate organic carbon (POC), and rates of sulfate reduction and oxygen uptake in sediments of the continental shelf of north Bering-Chukchi Sea. An additional goal is to develop a ‘working budget’ for components of the POC cycle for high and low productive regions of the Chukchi shelf. These data, in conjunction with additional in-put on fluxes and accumulation and burial rates of POC will provide a basis to establish a more comprehensive budget for comparison with other arctic margins and the global carbon budget.

Study Area and Environmental Setting

The study area comprises the relatively shallow (10-60 m deep) North Bering- Chukchi Sea, which is bounded by Russia in the west and Alaska in the east (Fig.1). Portions of the Bering Sea are biologically among the most productive regions of the world margins, supporting large bodies of commercial (above 50% of the U. S. Fishery) production and subsistence fisheries harvests. Many of the environmental features of the study area have been described (Sharma1979; Hood and Calder1981; Smith and Grebmeier 1995; Walsh et al.1989; Coachman and Hansell 1993; Feder et al. 1991a,b; Loughlin and Ohtani 1999, among others). Therefore, only a brief description of the area follows.

The hydrodynamics of the Northern Bering-Chukchi Sea region are complex. Water masses from various sources entrained in distinct currents maintain their identity along lengthy pathways. Mean current flow of Pacific-derived water is northward into the Arctic during most of the year. In the Bering and Chukchi Seas two Pacific-origin water masses, the Anadyr (AW) and Bering Shelf (BS) waters, are combined to form the modified Bering Shelf-Anadyr water (BSAW)), which is confined to the northwest Bering Sea (Fig. 1). Flowing eastward along the Alaskan coast is Alaska Coastal Water (ACW), which is heavily influenced by the Yukon River and numerous smaller drainages. In the summer the AW is relatively colder (<1.5oC), more saline (32.5) and carries higher concentrations of inorganic nutrients (NO3=>20 μM) than the ACW (T= o >2 C , S=<31.8, negligible NO3) or the BS (NO3= <1.0 μM) (Whitledge et al. 1988, 1992; Walsh et al., 1989; Feder et al., 1994; Cooper et al. 1997). The nutrient-rich upwelled water from the Bering Basin is advected onto the adjacent shelf and entrained in the AW as the Anadyr Stream (Fig.1). The BSAW and ACW move northward (net flow of 0.8 Sv in summer and 0.2 Sv in winter) into the Chukchi Sea, via the narrow (85 km) and shallow (54 m) Bering Strait (Fig.1). These two water masses are delineated by a sharp vertical hydrographic front (HF), which extends N-S from Chirikov Basin to southcentral Chukchi Sea (Fig.1). The BSAW plume, located west of the front and within Gulf of Anadyr, Chirikov Basin and Southern Chukchi Sea (Fig. 1), sustains high annual primary productivity (mean 470 g C m-2 y-1 and as great as 720-840 g C m-2 y-1, Springer and McRoy 1993). Farther north in Chukchi Sea the productivity to the west of the front

2 declines to ~ 250-300 g C m-2 y-1, whereas east of the front it is ~ 50-80 g C m-2 y-1 (Sambrotto et al.1984; Walsh et al.1989; Grebmeier et al.1995). However, there are large seasonal oscillations of nutrient concentrations (Whitledge and Luchin 1992) and productivity, with >70% of the annual productivity compressed within the episodic spring bloom, a phenomenon typical of the arctic seas (Dunbar 1968).There are several observations of East Siberian Current which promotes speculation about transport of POC into central Chukchi Sea and possibly into the Arctic Basin (Weingartner et al. 1999). Additional exchanges across the Chukchi shelf break may influence some water mass properties (Weingartner et al. 1998) but there is little evidence for offshore sources of DOM on the shelf.

Relatively large concentrations of ammonium have been observed on both the northern Bering and Chukchi shelves (Whitledge et al. 1988, 1992) which indicates relatively high rates of decomposition of organic matter (Walsh et al. 1981). The locations of the large ammonium concentrations south of St. Lawrence Island, the central Chirikov Basin, and north of Bering Strait are thought to be located over a series of sites with high POC in the sediments (Whitledge et al. 1992). Benthic production is closely coupled to pelagic production. A predominant portion of the phytodetritus, which is ungrazed in the water column settles on the sea floor, sustaining a rich macrobenthic community (Highsmith and Coyle 1992; Grebmeier 1993; Feder et al.1994; Grebmeier et al.1995; Grebmeier and Cooper1995). As a result, large populations of benthic–feeding marine mammals and birds serve as apex predators in the food chain (Grebmeier and Harrison 1992).

The eastern coastal region of the Northern Bering-Chukchi Sea is dominated by deltas of the Yukon, Kuskokwim, Noatak and Kobuk rivers; the hydrochemistry of these rivers are described in Telang et al. (1991). In the west the Anadyr is a minor river, which has minimal effects on the distribution of DOC in Anadyr Bay but may be a significant source of POC (Agatova et al. 1999). The nearshores east and south of the Seward Peninsula are characterized by numerous polynyas. Sea ice covers the north Bering- Chukchi Sea for 7-9 months, and its margins can sustain substantial production (Niebauer and Alexander 1985). The sea floor sediments of the Russian-Alaskan margin consist of a mosaic of poorly-sorted gravelly to muddy contemporary to relict sediments, displaying no spatial trend, and which are frequently reworked by ice gouging, marine mammals and occasional storm action (Naidu 1988).

7.2.2 Database, materials and methods

For this synthesis the original data for the concentrations of sediment particulate organic carbon (POC) and total nitrogen (N), C/N ratios (wt/wt), carbon isotope ratios (δ13C o/oo), flux and mass accumulation rates of total sediment and POC, percents organic carbon remineralization in sediment, sulfate reduction rates, and estimates of the benthic oxygen uptake rates are tabulated from several published papers (Grebmeier 1987, 1993; Grebmeier et al.1988, 1989; Feder et al.1994; Grebmeier and Cooper 1994, 1995; Baskaran and Naidu 1995; Naidu et al.2000; Cooper et al. 2002) and unpublished

3 reports (Feder et al. 1989, 1991). Our synthesis is based on analyses of about 400 surface grab sediment samples and selected gravity cores from the study area. The cores sectioned onboard the ship and the grab samples were kept frozen until analysis. The publications cited above also provide details on the methods of collection of bottom grab, core and sediment trap samples and the techniques utilized for the analyses of the above parameters. Therefore, only brief descriptions of the analytical methods are mentioned in this synthesis.

The concentrations of C and N for individual surface sediments of the Bering and Chukchi Sea are derived from several sources (Sharma 1979; Feder et al.1989, 1991; Grebmeier 1993; Grebmeier and Cooper 1995). Organic carbon and N contents on CO3- free sediments were determined either by an isotope ratio mass spectrometer or a P.E. Model 240B CHN analyzer (Naidu et al.1991). The C/N ratios are reported on a weight- to-weight basis of the POC and total N percents in the sediment organic matter (Chapter 1.4).

The carbon isotope ratios (δ13C, Chapter 1.4) of the above sediments were analyzed by three functionally similar mass spectrometers (V.G. Micro-mass Model 602E, Europa 20.20, and V.G. SIRA II), and are all referenced to the V-PDB standard, with a standard error of analysis of ±0.2 o/oo. The δ13C and C/N values for the North Bering-Chukchi Sea sediments are tabulated and their distribution illustrated in Naidu et al. (1993, 2000) and Grebmeier (1993), and updated here with supplemental new data.

The net mass sediment accumulation rates (Table 1) were determined by the 210Pb method, using alpha spectrometry (Baskaran and Naidu 1995). The sediment (total) and POC advective flux to the sea floor were estimated for selected 9 sites in the Chukchi Sea (Table 1) by collecting particles settling into a trap (designed after Hong 1986) deployed 5 m above the sea floor, in August when the sites were free of sea ice.

The mass accumulation rate of POC (Table 1) was ascertained from the product of the net accumulation (g cm-2 y-1) at a site and the POC concentration, mg/g, in the top 1-cm of sediment at that site (Baskaran and Naidu 1995). It was possible to estimate the percent of total POC remineralized in sediment only for two out of six locations investigated in the Chukchi Sea (Feder et al.1991). This study on two cores (SU5 and SU10) was over a core depth interval of 17cm, where net linear decreases in POC with core depths were observed (r:-0.88 and -0.90 for SU5 and SU10 respectively). This estimation was based simply on the difference between the total C concentration (on a dry weight basis) in the top 1-cm of a sediment core sample and the concentration of POC measured at depth in the core, following the ‘G’ model approach (Berner 1980, 1982; Martens and Klump, 1984). The POC burial rates at both stations were calculated following Gordon et al. (2001) from the product of the net accumulation rate at the individual core location and the minimum value of POC encountered at depth in the core (i.e., “refractory background” level of POC). The depth-integrated mineralization rate of POC was according to Marten and Klump (1984). Sediment sulfate reduction rates (mM m-2 d-1) were measured at six sites in the south Chukchi Sea (Feder et al.1991). These

4 measurements were collected on short (16-20 cm) HAPS core samples, using radiotracer 35 2- ( SO4 ) methodologies (Jorgensen 1978; Albert 1985).

-2 -1 Estimation of the benthic oxygen uptake rate (mM O2 m d ), which estimates the organic carbon utilization rate by benthos, was obtained in the North Bering-Chukchi Sea from 1984-1999 (Grebmeier1987, 1993, unpublished data; Grebmeier and McRoy 1989; Grebmeier and Cooper 1994,1995; Cooper et al. 2002). Each of the data points reported here for a site represents the mean of two replicate measurements conducted on HAPS (133 cm-2) cores incubated shipboard for approximately 12 hr at in-situ bottom temperatures (Grebmeier1988, 1989).

For estimation of sediment particle and POC flux sediment traps were deployed for about a month in summer at fairly widely spaced sites, and for a few days at one site (KW3) in winter. To collect the yearly fluxes of total particles and POC, the sediment traps ideally should have been deployed continuously throughout an annual cycle, but this was not possible due to logistical constraints. Another limitation in the estimation is that we assume that the particles intercepted in the traps are primary settling detritus advected vertically downward from ambient overlying waters containing insignificant amounts of resuspended particles from the sea bottom.

The data on the concentrations of mud, POC, and N, C/N, δ13C, and benthic oxygen uptake rates corresponding to sediments of the individual locations of the study area can be accessed at www.sfos.uaf.edu/pubs/naidu/.

7.2.2 Distribution and sources of organic carbon in surface sediments

. The concentrations of POC and N vary widely, ranging from 1.0 to 28.3 mg/g and 0.1 to 2.78 mg/g respectively (Fig.2). Generally, the concentrations of organic carbon in sediments of the North Bering-Chukchi are similar to those (5 to 15 mg/g) of most world shelves (Premuzic et al.1982; Romankevich 1984). The exceptions are northwest and southeast Gulf of Anydyr and northwest Chukchi Sea where locally higher (15-28 mg/g) C occur (Grebmeier 1993). Despite the large scatter in the POC vs. N plots (Fig.3) there is a net significant covariance between the two (p <0.05, n=104). There are no significant regional distributional trends for POC and N within the study area (Feder et al.1989, 1991). In the Chukchi Sea, where extensive set of samples have been analyzed for grain size and POC, we find a net significant correlation (p < 0.05, n=80) between POC and mud contents in sediments despite a large scatter in the plots.

The factors that control the organic carbon concentrations in marine sediments have been a topic of intense debate (e.g. Mayer 1994; Hedges and Keil 1995 among many others). In North Bering-Chukchi Sea we found no consistent across shelf distributional pattern in POC. It might have been expected that relatively higher concentrations of POC would occur in sediments underlying more productive waters, such as in the region west of the hydrographic front (marked HF on Fig. 1), where relatively higher fluxes of POC to the sea floor have been recorded (Tables 1and 2 ).

5 Furthermore, sediment grain size will also be an important factor controlling POC concentrations, based upon the covariance between sediment mud and C contents generally observed here and elsewhere (Blackburn 1987; Keil et al. 1994; Mayer 1994). On the North Bering-Chukchi Sea shelf a mosaic of sediment types (gravelly to muddy; Naidu 1984) occur. The large scatter in the POC-mud plots suggest that granulometry is not the sole factor influencing the sediment POC concentrations in the study area. It is possible that the effect of higher fluxes and deposition of POC is compensated by coarser sediments (probably low strand relict lag deposit), or that organic carbon remineralization rates and preservation efficiency are vary regionally among sediments with the same grain size distribution.

The sediment δ13C and C/N ratios have cross-shelf distribution trends. Generally, in the Bering Sea δ13C progressively increases from –24 o/oo in the coastal region in the east, especially off the Yukon River and Norton Sound, to –20 o/oo in the west within the Gulf of Anadyr (Fig. 4). Farther inshore along the Gulf margin and the Anadyr Bay and Anadyr Estuary the sediments are distinguished by the lowest δ13C values, (mean of 8 samples: -26.6 o/oo) (Naidu et al.1993, 2000). Likewise, in the Chukchi Sea a progressive increase in δ13C across the shelf is observed from the east to about the central shelf, and then farther west the values decrease to –22 o/oo or less near the East Siberian coast (Fig. 4). Within the entire study area a progressive cross-shelf decrease from the east to west in sediment C/N is clearly observed, with relatively higher values concentrated off major fluvial systems (Naidu et al. 1993; Fig. 5).

The sources of organic carbon and organic matter in sediments of north Bering- Chukchi Sea are discussed in detail by Blackburn (1987) and Naidu et al. (1993, 2000), using the distribution patterns in sediment δ13C and C/N as proxies (Stein, Chapter 1.4).These sets of data on δ13C and C/N have been supplemented since publication of the Naidu et al. (2000) work and updated versions are shown in Figures 4 and 5. Generally, in the North Bering Sea sediments there is an east to west, across shelf progressive increase in δ13C and a parallel decrease in C/N. In the Chukchi Sea a net decrease in δ13C and an increase in C/N occur from about mid shelf to the nearshore in east and west. Plots between δ13C versus C/N values for Chukchi Sea sediments indicate a significant correlation between the two (p <0.05, n=93). The lateral increase in δ13C and corresponding decrease in C/N are explained by a net increase in marine- versus terrigenous-dominated particulate organic carbon and organic matter respectively in sediments (Stein, Chapter 1.4). This conclusion is based on the presence in North Bering– Chukchi Sea of two end members, marine and terrigenous, with distinctly different δ13C signatures, -21.2 o/oo and –27 o/oo respectively (Naidu et al.1993, 2000). Therefore, a simple mixing equation (Calder and Parker1968; Fry and Sherr 1984; Stein, Chapter 1.4), considering the δ13C values of the end members and of sediment C for a location, can account the relative abundances of marine and terrigenous C at that location. Likewise, the sediment C/N is used to infer the sources of organic matter and by implication POC as well, assuming that the C/N end member values of marine and terrigenous organic matter are <5-7 and >15 respectively (Naidu et al.1993, 2000; Stein, Chapter 1.4). It is further suggested by the above authors, based on the sediment δ13C and C/N, that much larger rates of deposition of marine organic matter occur under relatively more productive

6 waters, such as within the region west of the hydrographic front (compare Figs.1, 4, and 5).

Recently, Schubert and Calvert (2001) found that significant quantities of bound inorganic nitrogen (Nbou, see Stein, Chapter 1.4) occur in Arctic Ocean deep-sea illitic sediments. Likewise, Muller (1977) reported large concentrations of ammonium ion adsorbed by clays in Pacific deep-sea sediments. These studies imply that the use of C/Ntotal instead of C/Norg (where Norg is Ntotal –Nbou, Chapter 1.4) may provide misleading assignments of the sources of marine sediment organic matter.While these studies were conducted on deep-sea sediments, it is possible that the high illite content of Bering– Chukchi Sea (Naidu et al. 1981) may preferentially adsorb ammonium ions (Nbou) and, thus, affect the C/N value (which is based on Ntotal). It is, therefore, possible that the lower C/N ratios in sediments generally in the region west of the hydrographic front in North Bering-Chukchi Sea (Fig. 5) are actually due to larger Nbou and not due to Norg of total N as has been implied relating to the organic matter sources (marine versus terrigenous). Plots of C versus N values (N=104) of the Chukchi Sea sediments show a significant covariance between the two, with the intercept at 0% of C corresponding to 0.107% N (Fig.3). This suggests that at least a portion of the total N in the sediment could be Nbou. However, it should be noted that in the Chukchi Sea sediments the concentrations of illite (and presumably Nbou) decrease from the nearshore to the central shelf (Naidu et al. 1981), a factor that would go against the possibility discussed above. We, suggest, therefore, that the C/Ntotal-based inferences on the provenances of organic matter in the north Bering-Chukchi Sea (Naidu et al. 1993, 2000) might be clarified by more detailed N partitioning investigations (Chapter 1.4) and additional biomarker studies (Hayes et al. 1989; Jasper and Gagosian 1989; Prahl and Muehlhausen 1989; Fahl and Stein 1999; Stein et al.1999; Stein, Chapter 1.4) could provide a more robust discrimination of organic carbon sources to the sediments in the Bering-Chukchi shelf.

7.2.4 Fluxes, Accumulation and Burial Rates and Remineralization of OC, and Benthic Oxygen Uptake Rates

It is assumed that the sediment mass and POC accumulation rates at the nine core sites (Table 1) are measures of the depositions of sediments and admixed POC on a constant depositional rate basis. Considering the shallow water depths of the shelf we suggest that in the Chukchi Sea the relative differences in the POC flux between locations reflect the primary productivity rates of overlying waters and the resulting mass of phytodetritus advecting down (Table 2). It should be noted that the yearly flux data (Tables 1 and 2) are computed by us from various one-time, short-term real field data that were reported on a daily rate basis by Baskaran and Naidu (1995). The computation assumed that the daily-based data is a representative mean only for approximately four months of the productive open water season, and that only 10% of the yearly fluxes of sediment and particulate POC occur during the rest of the eight months when primary production is negligible under sea ice cover (Baskaran and Naidu 1995).

7 Clearly, there is an across shelf east-west increase in the sulfate reduction rate (Table 1), with the lowest rate of sulfate reduction in Kotzebue Sound and the adjacent southeast Chukchi Sea region (e.g., Stations SU11, KS14, KS1 and KW3) and greatest sulfate reduction far offshore region (e.g., SU5 and SU8) in areas that also have high benthic biomass (Grebmeier et al.1988). These lateral variations in sulfate reductions in sediments are most likely due to a progressive east-west increased remineralization of resulting from greater deposition of labile, marine-derived organic carbon. Our findings are, therefore, consistent with Blackburn’s (1987) suggestion that there is a close correlation between microbial remineralization rates and the quality of organic matter and POC in sediments of Bering –Chukchi Sea.The benthic sediment oxygen uptake rates in the Chukchi shelf also show an across-the-shelf pattern, with values progressively increasing from the eastern nearshore to the mid shelf and then decreasing towards the East Siberian coast (Fig. 6). In the north Bering Sea the highest values are southwest of St. Lawrence Island and south of Bering Strait in Chirikov Basin, with values subsequently decreasing in a concentric pattern about the maxima (Fig. 6). The regional variations in the oxygen uptake rates are attributed primarily to the relative abundance of the benthic biomass, which in turn is determined by the lateral variations in marine vs. terrigenous POC and the total flux of POC depositing at the sea floor (Grebmeier et al. 1988).

As discussed earlier, it was possible to estimate the percentages of organic carbon mineralized and the burial rate of POC at two sites (SU5, a high productive site and SU10 a less productive site) in the Chukchi Sea (Tables 3 and 4). At four other sites investigated the POC was well mixed within the sediment cores precluding an estimate of the remineralization. Although the burial rate of POC at SU5 is about a factor of two higher than at SU10 site, the percent POC remineralized at these contrasting sites is similar (Table 1and 2). The regional difference in burial rate of POC is most likely due to the relatively higher accumulation rate of sediment and POC at the SU5 location.The burial efficiencies of POC (after Gordon et al. 2001) for SU10 and SU5 are 46 % and 64 % respectively. This between station difference is assigned mainly to the differences in gross sediment and POC accumulation rates, which is consistent with the widely held view for marine deposits (Muller and Suess 1979; Henrichs and Reeburgh 1987; Rullkotter 2000).

The regional variations in the rates of remineralization and burial of carbon and the sediment sulfate reduction, as discussed above, are consistent with the hypothesis formulated at the beginning of our investigations. Prior to the estimations of the rates, we had hypothesized that in Chukchi Sea the rates at productive sites in midshelf (e.g., SU5) would be significantly higher than in sediments at less productive sites in the nearshore (e.g., SU10). The underlying premise was that in highly productive waters relatively more marine-derived labile (more easily metabolizable) organic carbon will be deposited and that in less productive waters more land-derived refractory carbon will be accumulated (Blackburn 1987; Grebmeier 1987; Feder et al 1991; Meyers and Eadie 1993). The regional sediment δ13C and C/N values in sediment trap samples (Baskaran and Naidu 1995) and bottom sediments (Naidu et al. 1993, 2000) indicate that the POC

8 deposited at SU5 is indeed, as hypothesized, predominantly marine and, presumably therefore, relatively more labile than POC sedimenting at SU10.

Tentative Mass Balance of OC

Assuming SU5 and SU10, represent offshore high productive, and inshore, less productive portions of the Chukchi Sea respectively, our database in conjunction with the primary production rate provide a tentative basis to calculate aspects of the organic carbon mass balance for these regions of contrasting productivity (Fig. 7). A basic assumption in the ensuing calculation of the ‘working budget’ is that almost the entire initial source of POC is marine primary production (phytodetritus) from ambient waters overlying the above two sites. That this assumption is reasonable is supported by the C/N ratio (6-7) observed for suspended particles of surface and near bottom waters throughout the Chukchi Sea shelf (Feder et al. 1991). However, it should be noted that the nearshore North Bering-Chukchi Sea off the major rivers, receive significant inputs of land-derived POC from coastal erosion and/or fluvial outflow (Naidu et al.1993, 2000) as demonstrated by sediment δ13C and C/N values (Fig. 4 and 5). A potential complication to the budget calculation is that significant amounts of POC in the Chukchi Sea waters could be allochthonous marine POC, imported from the north Bering Sea (Walsh et al.1989; Feder et al.1994). However, no quantitative estimates are available on the import fluxes of terrigenous and allochthonous organic carbon and their distribution in Chukchi Sea.

It appears that about 90% of the POC derived from primary production from the productive region and about 74% from the relatively less productive waters settles to the sea floor. Further, only about 2% of the total organic carbon flux at SU5 and 65% at SU10 finally accumulate in the bottom surficial sediments.The rest of the POC reaching the sea floor must, therefore, be lost through resuspensionand advection, remineralization and/or uptake by benthic organisms. The significantly higher benthic biomass and benthic respiration rates at the more productive sites (Grebmeier 1987, Grebmeier etal. 1989; Fig. 6) imply that most of the settling POC escapes grazing by pelagic organisms, gets to the bottom and is eventually consumed by benthos. The subsequent losses due to remineralization of the sedimented POC at the SU5 and SU10 sites, are estimated to be 52% and 44% respectively. The depth-integrated remineralization rate at SU5 is higher by a factor of 2 than at SU10 (Table 2). Eventually, POC is buried at SU5 and SU10 sites at the rates of 1.02 mg cm-2 y-1 and 0.55 mg cm-2 y-1 respectively (Table 2, Fig. 7). A more comprehensive organic carbon budget for the Chukchi Sea incorporating allochthonous sources of marine and terrestrial POC, autochthonous production, export from the shelf and burial in shelf sediments must be established before valid comparisons can be made with other margins of the arctic (for example, Walsh and McRoy 1986; Grebmeier 1987; Fahl and Stein 1997; Macdonald et al.1998; refer also to Chapters 5 and 8). Progress towards this objective will also contribute to wider environmental research goals, such as assessing the potential role of sediments of the shelves and ocean basins as sinks for POC and in sequestration of natural and anthropogenic CO2 (Walsh et al. 1981; Berner 1992, Hedges et al. 1997; Naidu et al. 2000, de Haas et al. 2002).

9 7.2.5 Summary and conclusions

We have discussed here what little is known about the concentrations, sources, fluxes, accumulation and burial rates, and remineralization of POC of sediments of the North Bering-Chukchi Sea shelf. In the study area the POC concentrations range from 1.0 to 28.3 mg/g, which are close to the values generally reported for shelf sediments (5-15 mg/g). The POC distribution shows no across shelf pattern. Although there is a significant correlation in POC and mud contents, wide scatter in the binary plots suggests that grain size is not the sole factor determining sediment POC. Generally, an across shelf, from east to west, increase in δ13C and decrease in C/N in sediments are found. On the Chukchi Shelf on going from the low productive nearshore to the highly productive midshelf, there are also increases in the rates of POC remineralization, sulfate reduction and benthic oxygen uptake. The above trends reflect larger flux and deposition of marine POC relative to terrigenous POC in the midshelf.

A tentative budget for POC is formulated based on two sites, one representing the high productive (84 mg C cm-2 y-1) zone and the other representing low productive (7 mg C cm-2 y-1) zone (Fig.7). About 90% and 74% of the primary production of carbon from the productive and less productive waters, respectively settles to the bottom of which about only 2% and 65% respectively finally accumulate in the bottom sediments. The rest of the POC flux at the two sites is lost by advection, remineralization or consumption by benthic organisms. Subsequent remineralization of the accumulated POC is 52% and 44 % respectively. Further measurements are necessary to calculate a more comprehensive POC budget for the Alaskan arctic shelf.

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11 Hayes JM, Freeman KH, Popp BN, Hoham CH (1989) Compound-specific isotopic analyses: a novel tool for reconstruction of ancient biogeochmical processes. Adv Org Geochem 16:1115-1128 Hedges JI, Keil RG (1995) Sedimentary organic matter preservation: an assessment and speculative synthesis. Mar Chem 49:81-115 Hedges JI, Keil RG, Benner R (1997) What happens to the terrestrial organic matter in the ocean? Org Geochem 27:195-212 Henrichs SH, Reeburgh, WS (1987) Anaerobic mineralization of marine sediment organic matter: rates and the role of anaerobic processes in the oceanic carbon economy. Geomicrobiology J 5:191-237 Hong, GH (1986) Fluxes, dynamics and chemistry of particulate matter and nutrient regeneration in the central basin of Boca de Quadra, southeast Alaska. Ph.D. Thesis Univ. Alaska Fairbanks. pp.1-225 Hood DW, Calder JA (1981) The Eastern Bering Sea Shelf: oceanography and resources, vol 1. Univ Washington Press, Seattle, pp 1-625 Jasper JP, Gagosian RB (1989) Alkenone molecular stratigraphy in an oceanic environment affected by glacial freshwater events. Paleoceanography 4:603-614 Jorgensen BB (1978) A comparison of methods for the quantification of bacterial sulfate reduction in coastal marine sediments. Geomicrobiology 1:11-27 Keil RG, Tsamakis E, Fuh CB, Giddings C, Hedges JI (1994) Mineralogical and textural controls on organic composition of coastal marine sediments: hydrographic separation using SPLITT fractionation. Geochim Cosmochim Acta 57:879-893 Loughlin TR, Kiyotaka, O (eds) (1999) Dynamics of the Bering Sea. PICES/Univ Alaska Fairbanks, pp 1-825 Macdonald RW, Soloman SM, Cranston RE, Welch HE, Yunker MB, Gobeil C (1998) A sediment and organic carbon budget for the Canadian Beaufort Shelf. Mar Geol 144:255-273 Martens CS, Klump JV (1984) Biogeochemical cycling in an organic rich coastal marine basin.4. An organic carbon budget for sediments dominated by sulfate reduction and methanogenesis. Geochim Cosmochim Acta 48:1987-2004 Mayer LM (1994) Surface area control of organic carbon accumulation in continental shelf sediments. Geochim Cosmochim Acta 58:1271-1284 Mayer PA, Eadie BJ (1993) Sources, degradation and recycling of organic matter associated with sinking particles in Lake Michigan. Org Geochem 20:47-56 Muller PJ (1977) C/N ratios in Pacific deep-sea sediments: effect of inorganic ammonium and organic nitrogen compounds sorbed by clays. Geochim Cosmochim Acta 41: 765-776 Naidu AS (1988) Marine surficial sediments, section 4. In: Bering, Chukchi and Beaufort Seas, coastal and ocean zones strategic assessment data atlas. U.S. Department of Commerce, NOAA, Washington, DC Naidu AS, Creager JS, Mowatt TC (1981) Clay mineral dispersal patterns in the north Bering and Chukchi Seas. Mar Geol 47:1-15 Naidu AS, Scalan RS, Feder HM, Goering JJ, Hameedi MJ, Parker PL, Behrens EW, Caughey ME, Jewett SC (1993) Stable organic carbon isotopes in sediments of the north Bering-south Chukchi seas, Alaskan-Soviet Arctic shelf. Cont Shelf Res 13:669-691

12 Naidu AS, Cooper LW, Finney BP, Macdonald RW, Alexander C, Semiletov IP (2000) Organic carbon isotope ratios (δ13C) of Amerasian continental shelf sediments. Int J Earth Sci 89:522-532 Niebauer HJ, Alexander V (1985) Oceanographic frontal structure and biological production at an ice edge. Cont Shelf Res 4:367-388 Nihoul JCJ, Adam P, Brasseur P, Deleersnijder E, Djenidi S, Haus J. (1993) Three dimensional general circulation model of the northern Bering Sea’s summer ecohydrodynamcs. Cont.Shelf Res 13:509-542 Premuzic ET, Benkovitz CM, Gaffney JS, Walsh JJ (1982) The nature and distribution of organic matter in the surface sediments of world oceans and seas. Org Geochem 4:53-77 Prahl FG, Muehlhausen LA (1989) Lipid biomarkers as geo-chemical tools for paleoceanographic study. In: Berger WH, Smetacek VS, Wfer G (eds) Productivity of the oceans: present and past. Wiley, New York, pp 271-289 Rachold V, Grigoriev MN, Are FE, Solomon S, Reimnitz E, Kasens H, Antonow, M (2000) Coastal erosion vs riverine sediment discharge in the Arctic shelf seas. Int. J Earth Sciences 89:450-460 Rullkotter J (2000) Organic matter: the driving force for early diagenesis. In: Schulz HD, Zabel M (eds) Marine Geochemistry. Springer, Berlin, New York. pp129- 172 Romankevich EA (1984) Geochemistry of organic matter in the ocean. Springer, Berlin Heidelberg New York, pp1-334 Sambrotto RN, Goering JJ, McRoy CP (1984) Large yearly production of phytoplankton in the western Bering Strait. Science:1147-1150 Schubert CJ, Calvert SE (2001) Nitrogen and carbon isotopic composition of marine and terrestrial organic matter in Arctic Ocean sediments: implications for nutrient utilization and organic matter composition. Deep-Sea Res 48:789-810 Sharma GD (1979) The Alaskan Shelf: hydrography, sedimentary and geochemical environment. Springer, Berlin Heidelberg New York, pp 1-498 Shultz DJ, Calder JA (1976) Organic carbon 13C/12C variations in estuarine sediments. Geochim Cosmochim Acta 40:381-385 Smith OS Jr, Grebmeier JM (eds) (1995) Arctic oceanography: marginal ice zones and continental shelves. Am Geophys Union, Washington, DC, pp 231-261 Springer AM, McRoy CP (1993) The paradox of pelagic food webs in the northern Bering Sea—III. Patterns of primary production. Cont Shelf Res 13:575-599 Stein R, Fahl K, Niessen F, Siebold M (1999) Late Quaternary organic carbon and biomarker records from the Laptev Sea continental margin (Arctic Ocean): implications for organic carbon flux and composition. In: Kassens H, Bauch HA, Dmitrenko IA, Eicken H, Hubberten H-W, Melles M, Thiede J, Timokhov LA (eds) Land-ocean systems in the Siberian Arctic: dynamics and history. Springer, Berlin Heidelberg New York, pp 635-655 Telang SA, Pocklington R, Naidu AS, Romankevich EA, Gitelson II, Gladyshev MI (1991) Carbon and mineral transport in major North American, Russian Arctic, and Siberian Rivers: the St. Lawrence, the Mackenzie, the Yukon, the Arctic Alaskan Rivers, the Arctic Basin Rivers in the Soviet Union, and the Yenisei. In: Degens ET, Kempe S, Richey JE (eds) Biogeochemistry of major world rivers.

13 Wiley, New York, pp 75-104 Walsh JJ et al. (1989) Carbon and nitrogen cycling within the Bering/Chukchi Seas: source regions for organic matter affecting AOU demands of the Arctic Ocean. Prog Oceanogr 22:277-359 Walsh JJ, Rowe GT, Iverson RL, McRoy CP (1981) Biological export of shelf carbon is a sink of the global CO2 cycle. Nature 291:196-201 Walsh JJ, McRoy CP (1986) A tentative carbon (POC) budget for outer shelf and accumulation rate for continental slope, Bering Sea. Cont Shelf Res 5:259-288 Weingartner T, Cavalieri DJ, Aagaard K, Sasaki Y. (1998) Circulation, dense water formation, and outflow on the northeast Chukchi Shelf. J. Geophy Res 103:7647- 7661 Weingartner TJ, Danielson S., Saski Y, Pavlov V, Kulakov M (1999) A wind- and buoyancy-forced Arctic coastal current. J. Geophy Res 104:29697-29713 Whitledge TE et al. (1988) Biological measurements and related chemical features in Soviet and United States regions of the Bering Sea. Cont Shelf Res 8:1299-1319 Whitledge TE, Gorelbin MI, Cheryab SM (1992) Biogenic nutrient content. In: Nagel PA (ed) Results of the third joint US-USSR Bering and Chukchi Seas Expedition (BERPAC), summer 1988. USFWS, Washington DC, pp.39-49 Whitledge TE, Luchin VA (1999) Summary of chemical distributions and dynamics in the Bering Sea. In: Loughlin TR, Ohtani K (eds) Dynamics of the Bering Sea. Univ Alaska Press, Fairbanks, pp. 217-249

14 ______Table 1. Fluxes of total sediment (TSF) and particulate organic carbon (POCF), mass accumulation rates of sediments (MARS) and of POC (MAROC), percents POC remineralized (ROC), burial rates of POC (BROC) and sulfate reduction rates (SRR) at selected stations in Chukchi Sea (after Baskaran and Naidu 1995; Feder et al. 1989, 1991). All values, except ROC and SRR are in mg cm-2 y-1. The ROC and SRR are in percents and mM m-2 d-1 respectively.

Station Lat. Long. Water TSF POCF MARS MAR ROC BROC SRR N. W. Depth OC (m) KS1 66. 62o 163.00o 13 660 14.52 88 1.4 1.0 KW2 66.58o 163.00o 14 737 1.3 SU1 68.17o 168.00o 51 596 13.49 SU4 67.73o 166.00o 43 1517 24.73 SU8 66.52o 168.00o 34 4335 76.73 7.8 SU11 67.04o 165.75o 28 1036 25.07 31 0.1 2.9 KS14 66.83o 163.90o 24 686 88a 0.8 1.3 KS16 67.40o 164.45o 26 874 16.87 CH13 72.52o 164.13o 48 510 1.85 167 2.3 CH21 71.20o 164.13o 42 115 1.2 CH25 72.63o 167.08o 51 83 1.3 CH38 70.70o 167.38o 52 165 0.4 CH39 71.87o 168.26o 48 90 0.1 CH40 70.28o 167.90o 45 119 1.2 SU5 67.03o 169.00o 50 4335b 76.73b 159 1.6 52 1.02 23.6 SU10 70.15o 166.57o 42 510c 1.85c 119d 1.2 44 0.55 1.3 KW3 66.18o 162.00o 14 660 14.52 88a ______a, b, c, and d extrapolated from KS1, SU8, CH13 and CH40, respectively. For sites SU5, SU10 and KW3 the total sediment and POC fluxes, and for sites KS14 and KW3 the mass accumulation rates were unavailable. The values for these locations are extrapolated from adjacent stations, which have similar productive and depositional regimes (e.g. SU8 data is used for SU5, CH13 for SU10, and KS1 for KS14 and KW3).

15 ______

Table 2. Comparison of the Primary production rates (PPR), TSF, POCF, MARS, MAROC, ROC, BROC and DIOCMR for sites SU5 and SU10, which are representatives of relatively very productive and less productive regions, respectively, for Chukchi Sea. All values are in mg cm-2 y-1, except ROC and DIOCMR, which are in percents and mol m-2 y-1 respectively. ______

PARAMETER SU5 SU10 ______

Primary Production (PPR) 84a 7

Total sediment flux (TSF) 4335 510

Particulate organic carbon flux (POCF) 76 2

Mass accumulation rate of sediment (MARS) 159 119

Mass accumulation rate of POC (MAROC) 1.6 1.2

POC remineralized (ROC) 52 44

Burial rate of POC (BROC) 1.02 0.55

Sulfate reduction rate 23.6 1.3

Depth-integrated POC remineralization rate, 0.89 0.44 (DIOCMR) ______a: Based on the highest value reported (Springer and McRoy 1993)

16 List of Figures

Figure 1. Bathymetry, Hydrographic Front (HF), and major current systems in the north Bering-Chukchi Sea (after Coachman et al. 1975; Nihoul et al. 1993). ACW: Alaska Coastal water; BSC: Bering Slope Current; AW: Anadyr Water; BSAW: Bering Sea- Anadyr Water; ESC: East Siberian Current.

Figure 2. Distribution of particulate organic carbon, C (mg/g) in sediments of North Bering-Chukchi Sea.

Figure 3. The C (mg/g) versus N (mg/g) plots of sediments from Chukchi Sea.

Figure 4. Distribution pattern of δ13C o/oo in sediments of North Bering-Chukchi Sea.

Figure 5. Distribution pattern of C/N in sediments of North Bering-Chukchi Sea.

-2 -1 Figure 6. The benthic oxygen uptake rates (mM O2 m d ) across the shelf of North Bering-Chukchi Sea.

Figure 7. A schematic illustration of the mass balance of particulate organic carbon (POC) in sediments for the high and low productive zones of the Chukchi Sea shelf. PPR: primary productivity rate of POC; POCF: POC flux; MAOC: Mass accumulation rate of POC; OCM: POC lost by remineralization; BROC: POC burial rate. All values are in mg cm-2 y-1.

17 List of Tables

Table 1. Fluxes of total sediment ((TSF) and POC (POCF), mass accumulation rates of total sediments (MARS) and POC (MAROC), percent POC remineralized (ROC), burial rates of POC (BROC) and sulfate reduction rates (SRR) at selected stations in Chukchi Sea (after Baskaran and Naidu 1995; Feder et al., 1989 1991). All values except ROC and SRR are in mg cm-2 y-1. The ROC and SRR are in percents and mM m-2 d-1, respectively.

Table 2. Comparison of the Primary Production rates (PPR), TSF, POCF, MARS, MAROC, ROC, BROC and DIOCMR (Depth-integrated mineralization rate of POC) for two sites, SU5 and SU10, representatives of relatively high productive and less productive regions respectively for Chukchi Sea. All values are in mg cm-2 y-1, except ROC and DIOCMR, which are in percents and mol cm-2 y-1 respectively.

18 Sea Surface High Productivity Zone Low Productivity Zone

PPR: 84 PPR: 7

POCF: 76 POCF: 2

Sea Floor OCM: 0.83 MAOC: 1.6 MAOC:1.2 OCM: 0.52

BROC: ~1.02 BROC: ~0.55

Figure 7. Schematic illustration of the mass balance of POC for the high and low productive zones of the Chukchi Sea shelf. PPR: Primary productivity rate of POC; POCF: Particulate organic carbon flux; MAOC: Mass accumulation rate of POC; OCM: POC lost by remineralization; BROC: POC burial rate. All values are in mg cm-2 y-1