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Detection of rapid deposition of sea ice-rafted material to the Arctic Ocean benthos using the cosmogenic tracer 7Be
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Deep-Sea Research II 52 (2005) 3452–3461 www.elsevier.com/locate/dsr2
Detection of rapid deposition of sea ice-rafted material to the Arctic Ocean benthos using the cosmogenic tracer 7Be Lee W. Coopera,, Ingvar L. Larsenb, Jacqueline M. Grebmeiera, S. Bradley Moranc a
Department of Ecology & Evolutionary Biology, 569 Dabney Hall, University of Tennessee, Knoxville, TN 37830, USA b 98 Oklahoma Ave, Oak Ridge, TN 37830, USA c Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA Received 11 March 2004; accepted 11 September 2005 Available online 22 November 2005
Abstract During three icebreaker cruises in the Arctic Ocean under different sea-ice conditions in 2002, undisturbed benthic surface sediments were collected and assayed for the presence of a short-lived (t1/2 ¼ 53 d), particle-reactive cosmogenic radionuclide, 7Be, that is solely derived from atmospheric deposition. Under largely ice-covered conditions in May–June 2002, we did not detect this radionuclide in benthic surface sediments, despite significant inventories present in ice-rafted snow on the overlying sea ice (mean ¼ 86.8 Bq m2732.0 SD; n ¼ 9). During the July–August 2002 Shelf–Basin Interactions (SBI) cruise aboard the USCGC Healy and during a simultaneous cruise of the CCGS Sir Wilfrid Laurier on the Bering and Chukchi Shelf, which occupied the same general region following retreat and dissolution of Arctic ice cover, the 7Be present in this snow as well as surface deposition on to the sea ice-free water surface was detected in many benthic surface sediments, including some as deep as 945 m in Barrow Canyon. Inventories of 7Be in sediments were as high (60 Bq m2) as the entire decay-corrected inventory present earlier in some snow samples collected on the sea-ice cover. Other deposition indicators such as the inventories of sediment chlorophyll, sediment oxygen respiration rates and 234 Th-derived export fluxes also showed post-ice melt particle deposition and vertical transport, but in most cases the 7Be deposition was not tightly correlated with these other indicators, suggesting that 7Be sedimentation may not be controlled by the same processes. Our observations indicate that materials in sea ice, including contaminants, particulate organic, and mineral matter originating from atmospheric deposition or entrained in continental shelf sediments and rafted onto sea ice, can be rapidly transported to depth. The re-distribution of these materials as sea-ice drifts and eventually melts has the potential for impacting Arctic Ocean biogeochemical cycles and contaminant concentrations in areas of the Arctic remote from the original point of deposition. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sea ice; Beryllium-7; Cosmogenic isotopes; Rapid sedimentation; Arctic
1. Introduction
Corresponding author. Tel.: +1 865 974 2990; fax: +1 865 974 7896. E-mail address: [email protected] (L.W. Cooper).
0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.10.011
The exclusive natural source of the cosmogenic radionuclide 7Be is from production by cosmic ray spallation in the atmosphere (Arnold and Al-Salih, 1955). The radionuclide is deposited to the earth surface as an aerosol and with precipitation (Arnold
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and Al-Salih, 1955; Lal et al., 1958). In the few studies that have been accomplished in ice-covered seas, it has been expected that this radionuclide would be present in snow on sea-ice cover but not in the water column and underlying sediments in the absence of significant open-water (Grebmeier and Cooper, 1995; Kadko, 2000; Cooper et al., 2002; Eicken et al., 2002; Kadko and Swartz, 2004). In particular, Grebmeier and Cooper (1995) and Cooper et al. (2002) used 7Be as a marker for sedimentation within a polynya (open-water) and lead system in an otherwise ice-covered sea. In a similar way, the bomb fallout radionuclide 137Cs that is associated with clay particles in marine sediments was detected in portions of the Ross Sea where polynyas and other open water areas typically form each year but not under permanent ice cover (Grebmeier et al., 2003). In work we accomplished in 2002, as part of the Shelf–Basin Interactions (SBI) process study of Arctic shelf and basin exchange and also sampling associated with the Bering Strait Environmental Observatory on the Bering and Chukchi shelf, we used the receding ice pack in early summer as a presumed source of the radionuclide to underlying sediments. We expected that continuous sea-ice cover would prevent this atmospherically derived radionuclide from reaching benthic sediments prior to sea ice retreat. Because this radionuclide is comparatively short-lived (t1/2 ¼ 53 d), it would also provide an opportunity to observe seasonal deposition cycles as snow cover on ice melted and was introduced to the water column during sea-ice retreat and dissolution. Elemental beryllium has an affinity for particles and typical open-ocean profiles show preferential removal from surface waters (Measures and Edmond, 1982), so we expected that at least some 7Be would become attached to particles, whether in the melting snow, within sea ice, or in the surface water column, and be subsequently deposited to the sea floor. Our goal during two SBI process cruises was to document these short-term deposition events in conjunction with other indicators of recent particle deposition, such as changes in inventories of chlorophyll a in surface sediments (Cooper et al., 2002), sediment oxygen demand (Grebmeier and Cooper, 1995), as well as to compare the distribution of recently deposited 7Be to other tracers of particle export such as 234Th (Moran et al., 2005). It is worth pointing out that these particle export tracers and indicators do not necessarily record the same deposition processes as does 7Be. For example,
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sediment chlorophyll represents biologically based deposition from both sea-ice algae and watercolumn production, and sediment oxygen demand is a time-delayed response to recent biological deposition. The export flux of 234Th is also controlled by water-column processes (reviewed by Moran et al., 2003). By comparison, 7Be is best understood as a short-term tracer of particlereactive materials present within snow cover on the sea-ice surface, so our goal was as much to understand the differences as well as similarities to other sedimentation processes. 2. Methods Snow samples (0.0625 m2) were collected from the surface of sea ice during the early season research cruise (May–June 2002) prior to any significant melt (H. Eicken, pers. comm.). Care was taken to collect all snow present above the sea-ice surface. Snow was brought aboard ship, melted and shipped to the University of Tennessee for gamma spectroscopy. All counting was completed within 3 months of collection (2 half-lives); many samples were transported by helicopter to shore prior to the end of each cruise to facilitate faster initiation of gamma spectroscopy of the short-lived radionuclide. Prior to counting, liquid (snow) samples were concentrated by heating; weak acid (5% HCl) was used to keep 7Be in solution and prevent adherence to container walls. Precipitation samples collected during the summer (July–August 2002) cruise were handled similarly. Two precipitation collectors were mounted on the flying bridge of the USCGC Healy during the time that sediment sampling was underway, and precipitation flux inventories of 7Be during the cruise were determined by concentration and acid washing of the liquid precipitation samples as with the melted snow samples. These atmospheric flux data were extrapolated to a square meter basis, based upon the surface area of the precipitation collector, assuming steady-state deposition over the integrated collection period. Gamma spectroscopy of all samples was performed using a Canberra GR4020/S reverse electrode closed-end coaxial detector at the University of Tennessee. 7Be was detected by gamma decay at 477.6 KeV, using Canberra Genie 2000 software, which included corrections for efficiency and calibration with standards traceable to the US National Institute of Science and Technology. Surface sediments on all three cruises (two on the USCGC Healy and one on
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the CCGS Sir Wilfrid Laurier) were collected using either a single core or multi-core version of a 133cm2 HAPS corer (Kanneworff and Nicolaisen, 1973). Sediments were sectioned to depths of up to 15 cm, although no 7Be was detected below the 1-cm increment in any sample. Ninety cubic centimeters of each 1-cm sediment increment were added to aluminum cans with calibrated geometries for direct counting, using the same gamma spectroscopy system. When 7Be was not detected in surface sediments in the 90 cm3 cans, a larger 1-l sample of surface sediments from the same location, collected using a 0.10-m2 van Veen grab was counted, using 0.5- or 1-l Marinelli beakers with calibrated geometries. Since the van Veen grab penetrates to variable depths, and does not return entirely undisturbed surface sediments to the surface, it was not possible to accurately quantify the inventory of 7Be measured in these samples on a squaremeter basis, and we characterize these samples (not detectable in cores, but detectable in larger grab samples) as having ‘‘trace’’ inventories of 7Be. Despite the less quantifiable data obtained from the van Veen grab, use of this secondary sediment collection tool permits detection of the radionuclide in surface sediments at higher sensitivities because of the much larger volume of sample assayed. Samples for determination of chlorophyll a in surface (0–1 cm) sediments were also collected from the top 0–1 cm of the multi-HAPS cores. Following dark storage for 12 h in 90% acetone, chlorophyll a concentrations associated with small surface area aliquots of sediments were determined fluorometrically (Cooper et al., 2002). Other short-term
sedimentation data, including 234Th derived export fluxes and sediment oxygen respiration rates were drawn from collaborating studies also accomplished on the two USCGC Healy cruises (Moran et al., 2005; J. Grebmeier, unpublished data). 3. Results We determined the inventory of 7Be present in snow deposited on sea-ice cover over the Arctic shelf, slope and deep oceanic basin during the SBI Process cruise of the USCGC Healy in May–June 2002 (Table 1). In follow-up cruises of the Healy and the CCGS Sir Wilfrid Laurier in July–August, 2002, we determined the inventories of the radionuclide present in the top 1 cm of sediment cores collected in the same region following the retreat of sea ice (Table 2). As expected, because the continuous sea-ice cover present in the spring prevented the atmospherically derived radionuclide from reaching the benthic sediments prior to sea ice retreat, 7Be was not detectable in any benthic sediments sampled in May–June 2002, except for trace activity at Station HV1, immediately north of the Bering Strait, while the study area was covered with seasonal ice cover. The shipboard precipitation flux during the second cruise of the Healy, under open-water conditions, decay-corrected to 1 August 2002, was similar to the inventory present in snow on the sea ice during the first cruise (Table 1). When 7Be was detected in Marinelli beakers, but not in the 90-cm3 cans collected at the same location, these data are indicated as having had a trace of 7Be detected, as described in Section 2 (Table 2).
Table 1 Inventories of 7Be in surface snow and precipitation flux during second cruise of the USCGC Healy Location
Activity, 7Be (Bq m2)7SE at date of collection
Date of collection
Activity, 7Be (Bq m2) corrected to 1 August 2002
73.341N, 160.351W 73.741N, 158.941W 73.451N, 157.571W 73.101N, 158.211W 72.871N, 158.551W 72.611N, 158.771W 72.251N, 159.851W 71.491N, 153.901W 72.071N, 154.461W Precipitation flux Precipitation flux
94.278.0 58.874.4 99.470.4 119.5713.0 41.376.3 119.4711.2 81.277.0 122.377.0 45.277.9 32.572.4 35.472.9
20 May 2002 21 May 2002 23 May 2002 24 May 2002 27 May 2002 29 May 2002 30 May 2002 3 June 2002 5 June 2002 19 July–21 August 2002 19 July–21 August 2002
36.4 23.0 40.0 48.7 17.5 51.9 35.8 56.8 21.6 32.5 35.4
The decay-corrected values were calculated using ln(2)/(53)*difference (days) between the date of collection and a common date, 1 August 2002, that would permit comparison among samples collected on various dates.
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Table 2 Inventories of 7Be in surface (0–1 cm) sediments Station number, location
Date of collection
7
Be activity at date of collection (Bq m27SE)
Water depth (m)
Healy-12, 71.421N, 157.401W Healy-13, 71.641N, 155.901W Healy-14, 71.921N, 154.911W Healy-16, 72.181N, 153.911W Healy-17, 72.511N, 153.601W Healy-19, 71.961N, 152.131W Healy-20, 71.701N, 152.431W Healy-21, 71.641N, 152.381W Healy-22, 71.551N, 152.291W Healy-23, 71.441N, 152.461W Healy-24, 71.281N, 152.571W Healy-25, 72.231N, 159.331W Healy-26, 72.591N, 158.641W Healy-27, 72.711N, 158.521W Healy-35, 73.221N, 160.161W Healy-37, 73.051N, 160.421W Healy-38, 72.981N, 160.601W Healy-39, 72.731N, 161.321W Healy-41, 73.111N, 162.941W Healy-43, 73.621N, 165.471W Healy-44, 73.341N, 165.611W Healy-45, 70.661N, 166.561W Laurier-UTBS4, 64.961N, 169.881W Laurier-UTBS1, 64.991N, 169.141W Laurier-UTN1, 66.711N, 168.401W Laurier-UTN2, 67.051N, 168.731W Laurier-UTN3, 67.331N, 168.991W Laurier-UTN4, 67.501N, 168.911W Laurier-UTN5, 67.671N, 168.961W Laurier-UTN6, 67.741N, 168.441W Laurier-UTN7, 68.001N, 168.931W
21 July 2002 22 July 2002 23 July 2002 25 July 2002 27 July 2002 31 July 2002 1 August 2002 2 August 2002 3 August 2002 4 August 2002 4 August 2002 6 August 2002 7 August 2002 7 August 2002 16 August 2002 17 August 2002 18 August 2002 18 August 2002 19 August 2002 19 August 2002 20 August 2002 21 August 2002 17 July 2002 17 July 2002 17 July 2002 18 July 2002 18 July 2002 18 July 2002 18 July 2002 18 July 2002 18 July 2002
14.8711.7 18.875.6 17.776.5 n.d n.d. n.d. 12.9711.0 54.8710.8 Trace Trace 6.3717.6 16.276.0 Trace Trace n.d. Trace 15.776.3 60.2711.3 Trace 0.971.5 Trace 27.9750.3 31.8711.3 Trace 60.878.0 29.7734.7 Trace Trace n.d. Trace n.d.
127 162 563 1863 3197 2133 945 407 185 127 49 48 88 206 477 189 89 51 151 121 72 50 72 47 34 47 51 51 52 51 59
n.d. ¼ not detected; trace ¼ not detected in 90 cm3 aliquots collected using corer, but 7Be could be detected in 0.5-l Marinelli beakers collected using a 0.10 m2 van Veen grab, which were not quantifiable as to sediment surface area for the top 1 cm. A trace of 7Be was detected at the location of Laurier-UTN4 in May 2002, which was sampled from the USCGC Healy.
Decay-correction of the 7Be snow inventory in May–June 2002 to a common date of 1 August 2002 indicated that as much as 100% (60 Bq m2 as of 1 August 2002) of the 7Be inventory present in snow in May–June 2002 had reached surface sediments by the time of our sediment sampling in July–August 2002 (Table 2). This short-lived radionuclide was detected at depths as great as 945 m in Barrow Canyon (BC) to the north of Alaska, although it was also not detected in some shallower, inshore sediments (Fig. 1), showing an uneven pattern of particle deposition, with deposition apparently enhanced in portions of the Barrow submarine canyon (BC stations) and to the immediate east on the East Barrow (EB) transect line.
Other indicators of particle deposition, including sediment chlorophyll inventories, sediment oxygen respiration, and 234Th-derived export fluxes were evaluated in conjunction with these 7Be data. Sediment chlorophyll a inventories, for example, increased between the May–June and July–August 2002 cruises (Fig. 2). For the same stations occupied on both cruises, sediment chlorophyll a inventories on each cruise are correlated, although at the higher sediment chlorophyll a inventories (higher deposition), summer inventories fall well above a 1:1 ratio relative to spring inventories (Fig. 3), indicating enhanced deposition between the May–June 2002 and the July–August 2002 sampling.
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Fig. 1. 7Be fluxes from surface of sea ice to surface sediments of Arctic shelf and slope. Star symbols indicate inventory in snow on sea-ice cover, 20 May to 5 June 2002. Darker circles indicate an inventory was measurable on surface (0–1 cm) sediments. Lighter circles indicate trace activities present in surface sediments in Van Veen grab (1 l assayed), 21 July to 21 August 2002, but that the radionuclide was not detected in core tops (90 cm3 assayed) sampled at the same stations. White circles indicate locations where 7Be was not detected in core tops or grabs. White squares indicate locations where cores where collected, but samples were not assayed for 7Be. Gray tones shown reflect bathymetry, and where numbered, are depth (m).
234
Th-derived export fluxes and sediment oxygen respiration rates were also higher immediately before or during the July–August cruises (Moran
et al., 2005; J. Grebmeier, unpublished data) including enhanced deposition in BC and portions of the EB line where the highest 7Be fluxes also
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Fig. 2. Inventories of chlorophyll a (mg m2) present in surface (0–1 cm) sediments at tops of HAPS cores during three cruises in 2002. (A) May–June cruise of the USCGC Healy, (B) July–August cruises of the USCGC Healy and CCGS Sir Wilfrid Laurier.
were detected in surface sediments (Fig. 1; Table 2). Despite this, there were no statistically robust relationships between these alternate deposition indicators and the activities of 7Be that were detected in surface sediments on the July–August
2002 cruise (Fig. 4). In addition to the thoriumderived carbon fluxes (Fig. 4), we found no correlation between 7Be and the directly measured 234 Th flux data (Moran et al., 2004) from surface waters (data not shown).
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40
summer sediment chlorophyll a (mg m-2)
BC2 35
post-sea ice melt deposition 30
1:1 ratio (steady-state deposition)
WHS2
25
WHS1
20 15 10 5 0 0
5
10
15
20
25
spring sediment chlorophyll a (mg
30
35
40
m-2)
Fig. 3. Relationship between chlorophyll a in surface sediments collected on the May–June 2002 USCGC Healy cruise versus surface sediments collected at the same stations on the July–August 2002 cruises of the USCGC Healy and CCGS Sir Wilfrid Laurier.
4. Discussion We did not detect 7Be in any benthic sediments sampled in May–June 2002, except a trace activity in one station (HV1) immediately north of the Bering Strait, while the Arctic shelf, slope and deep basin was covered with seasonal ice cover. Consequently, virtually all 7Be observed in surface sediments in July–August 2002 was deposited during the time interval in summer before and during the second cruise as sea ice dissipated in the SBI study area. Stable oxygen isotope data indicate that freshwater contributed from melted sea ice was widespread in the study area by August 2002 replacing runoff that was almost exclusively the freshwater component source in May–June 2002 (Cooper et al., in press). Precipitation fluxes were similar during the July–August 2002 cruises as were existing inventories of 7Be in snow on surface sea ice, so it is not possible to rule out continuing deposition after sea ice melt from 7Be deposited to the sea surface in July–August 2002. However, other observations indicate that the earlier dissolution of sea ice was a key deposition precursor. For example, observations in May–June 2002 indicated that sea ice that was sampled in the Chukchi and Beaufort Seas carried significant sea-ice algal communities as well as ice-rafted sediments (H. Eicken, unpublished
data) that were entrained during sea-ice formation on continental shelves. It is possible that both sea-ice algae and ice-rafted sediments were the carrier mechanisms for rapid transport of the radionuclide deposited in snow to the sea floor. The inventory of chlorophyll a present in surface sediments increased between the spring and summer cruises (Fig. 2) but sea-ice algal communities were primarily present on the underside of the ice surface and may have been sedimented into the water column prior to coming in contact with 7Be deposited in melting snow on the sea-ice surface. The lack of any statistically robust relationships between the inventories of 7Be present at specific stations and sediment chlorophyll a, sediment oxygen respiration rates and 234Th-derived export fluxes also suggest that 7Be deposition may not be as strongly linked to sedimentation from marine phytoplankton production (water column or sea ice), but was rather tied to mineral-based sedimentation from sea ice. Although we were not able to explicitly test for mineral-based sedimentation, our rationale for this hypothesis is as follows. Most of the 7Be is associated with snow at the sea surface and most of the chlorophyll is associated with the ice-seawater interface on the underside of the sea ice. Since the sea ice carries ice-rafted sediments, and 7Be is known to be particle-reactive,
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sediment O2 respiration (mmol m-2 d-1)
sediment chlorophyll (0-1 cm; mg m-2)
40 35 30 25 20 15 10 5 0 40 35
10
20
30
40
50
60
70
10
20
30
40
50
60
70
0 7Be not 10 detected
20
30
40
50
60
70
30 25 20 15 10 5
234
Th-derived POC flux (mmol m-2 d-1)
50 40 30 20 10
80
7Be (Bq m-2)
Fig. 4. Inventories of 7Be in surface sediments (0–1 cm) collected on the July–August 2002 cruises of the USCGC Healy and CCGS Sir Wilfrid Laurier relative to samples collected at the same stations: (A) sediment chlorophyll (0–1 cm); (B) sediment oxygen respiration rates measured in shipboard incubations; (C) particle export, as estimated through 234Th and particulate organic carbon measurements using in-situ pumping methodologies. Additional details on sampling and methods for sediment oxygen respiration and 234Th assays are available elsewhere (Cooper et al., 2002; Moran et al., 2005).
it makes sense that the 7Be would come into contact with sediments in the ice prior to ice algae on the underside of sea ice. Consequently, it seems most likely that 7Be deposited in snow on the sea-ice surface infiltrated into channels as the surface snow cover and sea ice melted in early June 2002 (H. Eicken, pers. comm.), and the radionuclide became bound to ice-rafted sediments when in contact with sediments. Following further ice dissolution, the 7Be-bound sediments were then rapidly deposited to the ocean floor. It is worth noting that another study of 7Be sedimentation from multi-year ice during the SHEBA program indicates that a significant fraction of the ice-rafted radioisotope inventory can be
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trapped in melt ponds and not reach the upper mixed layer of the ocean (Eicken et al., 2002). However, in our study that was undertaken in much thinner seasonal ice, the sea ice present in May–June 2002 had essentially melted in place by July–August 2002, which is confirmed by the oxygen isotope composition of the surface ocean layers during the later sampling (Cooper et al., in press). Consequently, unlike at SHEBA, no ice was left for the 7 Be inventory to infiltrate into, which set up the highly efficient transfer of the radioisotope to the sea floor that we observed. Our examination of the linkage between 7Be deposition and other indicators of deposition from biological production (i.e., sediment chlorophyll a, sediment oxygen respiration rates and 234Th-derived export fluxes) should not be considered conclusive, however. Several factors limited our analyses, and it is noteworthy that the highest 7Be inventories were observed in regions such as BC and the slope portions of the EB transects (Table 2) where there were also indications of high biologically based sedimentation (Fig. 3; see also Moran et al., 2005; J. Grebmeier, unpublished data). Since these and other SBI studies are showing evidence of the importance of BC and ‘‘downstream’’ continental slope portions of the EB transects for deposition of organic materials, the same particle focusing processes may also influence the settling of sea ice rafted sediments. This also might explain why some of the higher 7Be activities were observed in relatively deep water and not necessarily on the shallow shelves where vertical transport distances from the sea surface are shorter. Our data analyses were limited by relatively high counting errors for 7 Be (Table 2) and the number of locations where no 7 Be was detected, but where indicators such as sediment chlorophyll a could be reproducibly measured (e.g., Figs. 3 and 4). This suggests that relatively low sensitivity for 7Be detection as well as the magnitude of counting errors were in part responsible for the low degree of correlation between 7Be inventories and other deposition indicators. Chlorophyll a also has a much longer biological half-life in sediments (Itakura et al., 1997; Lewis et al., 1999); we observed that it was present in sediments during the May–June 2002 cruise when there were no indications of high contemporaneous sedimentation. 234Th-derived export flux data also were limited in this study to the water column (Moran et al., 2005). Although both 234Th and 7 Be are gamma emitters, self-absorption of the
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energetically weak gamma-ray associated with 234Th decay in the calibrated containers we used that were necessary to sensitively assay 7Be prevented us from obtaining data on excess (not supported) 234Th present in surface sediments. Because of the short half-lives of both 234Th and 7Be (24 and 53 d, respectively), significant additional radioisotope counting instrumentation will have to be available to conclusively determine the differences in the relationships among 234Th and 7Be inventories in sediments and marine sedimentation processes. Our results do provide indications that particlereactive material, including contaminants such as heavy metals, organic materials, and radionuclides present in sea ice cover are likely to become bound to larger particles as sea ice melts and will be quickly deposited to the sea floor. These findings have consequences for biogeochemical cycles and food chains in Arctic systems. The benthos on various Arctic shelves are considered particularly important as food sources for benthic-feeding whales, walrus, bearded seals, and diving ducks (Grebmeier et al., 1995). There is, moreover, evidence for an increasingly seasonal pattern of sea-ice cover in the Arctic as the more permanent ice retreats (e.g., McPhee et al., 1998; Rothrock et al., 1999). Thus, contaminants deposited on Arctic sea ice will be more quickly incorporated into surface sediments and potentially into food webs. Particularly in areas of the Arctic where sea ice melts, such as Fram Strait, higher degrees of deposition of sea ice-rafted materials, including contaminants, is to be expected (Pfirman et al., 1995; Cooper et al., 1998; Masque´ et al., 2003). These findings also support the hypothesis (Cooper et al., 2000) that deposition from sea-ice cover is the mechanism responsible for sedimentation of plutonium with isotopic ratios consistent with near-field tropospheric, rather than global stratospheric bomb fallout in much of the deep Arctic Ocean. The Arctic Ocean has very low inventories of plutonium present in deep-sea Arctic sediments, but somewhat paradoxically, over much of the deep Arctic basin, the proportion of the fissionable isotope 239Pu is comparatively high (i.e., plutonium isotope ratios of 240Pu/239Pu are low), indicating that the origin of the plutonium is not simply stratospheric bomb fallout as is the case in most of the rest of the world (Cooper et al., 2000; Smith et al., 2000) The same rapid deposition mechanism from the atmosphere to the benthos that we observed here for 7Be also could have
played a role in the rapid deposition of tropospheric bomb fallout from Arctic nuclear tests such as on Novaya Zemlya tests during the late 1950s to early 1960s. While other sources are possible, such as reprocessed nuclear fuel cycle waste, regional distribution of near-field, low-yield tropospheric bomb fallout on Arctic sea ice might have been the source of the low 240Pu/239Pu ratio material that is found in deep Arctic Ocean sediments. Sedimentation from sea ice could have led to the widespread distribution of plutonium with comparative low ratios of 240Pu/239Pu ratios in much of the deep Arctic Ocean.
5. Conclusions Sampling of benthic surface sediments in May– June 2002 and July–August 2002 in shelf and slope regions of the Arctic Ocean indicate that a high proportion, up to 100%, of the cosmogenic radionuclide 7Be present on the sea-ice surface prior to snowmelt and sea-ice dissolution, is deposited to benthic sediments within 3 months. Deposition was detected to depths as great as 945 m, and the geographical patterns of distribution showed that deposition was highest in shelf regions, and in some deeper waters in BC. Nevertheless, deposition of 7Be was not conclusively linked to other deposition indicators of biological production, including 234Th export, 234Th-derived carbon export fluxes, sediment chlorophyll a, and sediment oxygen uptake, suggesting that the deposition may be primarily driven by mineralogical attachment of 7Be to sea ice-rafted sediments.
Acknowledgements We thank Hajo Eicken and Rolf Gradinger for collecting the snow samples, and Jim Bartlett, Betty Carvellas, Jacyln Clement, Pat Kelly, Rick Nelson, Sandor Mulsow and the marine science technician team aboard the USCGC Healy and the deck crew aboard the CCGS Sir Wilfrid Laurier for field assistance in collecting samples. Two anonymous reviewers, Hajo Eicken and editor Rodger Harvey provided comments that helped improve earlier versions of the manuscript. Financial support was provided to the Shelf-Basin Interactions and Bering Strait Environmental Observatory projects by the US National Science Foundation.
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References Arnold, J.R., Al-Salih, H.A., 1955. Beryllium-7 produced by cosmic rays. Science 121, 451–453. Cooper, L.W., Larsen, I.L., Beasley, T.M., Dolvin, S.S., Grebmeier, J.M., Kelley, J.M., Scott, M., Johnson-Pyrtle, A., 1998. The distribution of radiocesium and plutonium in sea ice-entrained Arctic sediments in relation to potential sources and sinks. Journal of Environmental Radioactivity 39, 279–303. Cooper, L.W., Kelley, J.M., Bond, L.A., Orlandini, K.A., Grebmeier, J.M., 2000. Sources of the transuranic elements plutonium and neptunium in Arctic marine sediments. Marine Chemistry 69, 253–276. Cooper, L.W., Grebmeier, J.M., Larsen, I.L., Egorov, V.G., Theodorakis, C., Kelly, H.P., Lovvorn, J.R., 2002. Seasonal variation in sedimentation of organic materials in the St. Lawrence Island polynya region, Bering Sea. Marine Ecology Progress Series 226, 13–26. Cooper, L.W., Benner, R., McClelland, J.W., Peterson, B.J., Holmes, R.M., Raymond, P.A., Hansell, D.A., Grebmeier, J.M., Codispoti, L.A., in press. Linkages among runoff, dissolved organic carbon, and the stable oxygen isotope composition of seawater and other water mass indicators in the Arctic Ocean. Journal of Geophysical Research Biogeosciences. Eicken, H., Krouse, H.R., Kadko, D., Perovich, D.K., 2002. Tracer studies of pathways and rates of meltwater transport through Arctic summer sea ice. Journal of Geophysical Research 107, 8046 10.1029/2000JC000583. Grebmeier, J.M., Cooper, L.W., 1995. Influence of the St. Lawrence Island polynya upon the Bering Sea benthos. Journal of Geophysical Research 100, 4439–4460. Grebmeier, J.M., Smith, W.O., Conover, R.J., 1995. Biological processes on Arctic continental shelves: ice–ocean–biotic interactions. In: Smith, W.O., Grebmeier, J.M. (Eds.), Arctic Oceanography: Marginal Ice Zones and Continental Shelves. American Geophysical Union, Washington, DC, pp. 231–261. Grebmeier, J.M., DiTullio, G.R., Barry, J., Cooper, L.W., 2003. Benthic carbon cycling in the Ross Sea polynya, Antarctica: benthic community metabolism and sediment tracers. In: DiTullio, G.R. (Ed.), Biogeochemical Cycles in the Ross Sea, pp. 313–326. Itakura, S., Imai, I., Itoh, K., 1997. ‘‘Seed bank’’ of coastal planktonic diatoms in bottom sediments of Hiroshima Bayt, Seto Inland Sea, Japan. Marine Biology 128, 497–508. Kadko, D., 2000. Modeling the evolution of the Arctic mixed layer during the fall 1997 Surface Heat Budget of the Arctic
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Ocean (SHEBA) Project using measurements of 7Be. Journal of Geophysical Research 105, 3369–3378. Kadko, D., Swartz, P., 2004. The source of the high heat and freshwater content of the upper ocean at the SHEBA site in the Beaufort Sea in 1997. Journal of Geophysical Research 109, 7 1029/2002JC001734. Kanneworff, E., Nicolaisen, W., 1973. The ‘‘HAPS:’’ a frame supported bottom corer. Ophelia Supplement 10, 119–129. Lal, D., Malhotra, K., Peters, B., 1958. On the production of radioisotopes in the atmosphere by cosmic radiation and their application to meteorology. Journal of Atmospheric and Terrestrial Physics 12, 306–328. Lewis, J.B., Harris, A.S.D., Jones, K.J., Edmonds, R.L., 1999. Long-term survival of marine planktonic diatoms and dinoflagellates in stored sediment samples. Journal of Plankton Research 21, 343–354. McPhee, M.G., Stanton, T.P., Morison, J.H., Martinson, D.G., 1998. Freshening of the upper ocean in the Central Arctic: is perennial sea ice disappearing? Geophysical Research Letters 25, 1729–1732. Masque´, P., Cochran, J.K., Hebbeln, D., Hirschberg, D.J., Dethleff, D., Winkler, A., 2003. The role of sea ice in the fate of contaminants in the Arctic Ocean: plutonium atom ratios in the Fram Strait. Environmental Science and Technology 37, 4848–4854. Measures, C., Edmond, J.M., 1982. Beryllium in the water column of the central North Pacific. Nature 297, 51–53. Moran, S.B., Weinstein, S.E., Edmonds, H.N., Smith, J.N., Kelly, R.P., Pilson, M.E.Q., Harrison, W.G., 2003. Does 234 Th/238U disequilibrium provide an accurate record of the export flux of particulate organic carbon from the upper ocean. Limnology and Oceanography 48 (3), 1018–1029. Moran, S.B., Kelly, P., Hagstrom, K., Smith, J.N., Grebmeier, J.M., Cooper, L.W., Cota, G.F., Walsh, J.J., Bates, N.R., Hansell, D.A., Maslowski, W., 2005. Seasonal changes in POC export flux in the Chukchi Sea and implications for water column-benthic coupling in Arctic shelves. Deep Sea Research II, this issue [doi:10.1016/j.dsr2.2005.09.011]. Pfirman, S.L., Eicken, H., Bauch, D., Weeks, W.F., 1995. The potential transport of pollutants by Arctic sea ice. The Science of the Total Environment 159, 129–146. Rothrock, D.A., Yu, Y., Maykut, G.A., 1999. Thinning of the Arctic sea-ice cover. Geophysical Research Letters 26, 3469–3472. Smith, J.N., Ellis, K.M., Polyak, L., Ivanov, G., Forman, S.L., Moran, S.B., 2000. 239,240Pu transport into the Arctic Ocean from underwater nuclear tests in Chernaya Bay, Novaya Zemlya. Continental Shelf Research 20, 255–279.
Deep-Sea Research II 52 (2005) 3452–3461 www.elsevier.com/locate/dsr2
Detection of rapid deposition of sea ice-rafted material to the Arctic Ocean benthos using the cosmogenic tracer 7Be Lee W. Coopera,, Ingvar L. Larsenb, Jacqueline M. Grebmeiera, S. Bradley Moranc a
Department of Ecology & Evolutionary Biology, 569 Dabney Hall, University of Tennessee, Knoxville, TN 37830, USA b 98 Oklahoma Ave, Oak Ridge, TN 37830, USA c Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA Received 11 March 2004; accepted 11 September 2005 Available online 22 November 2005
Abstract During three icebreaker cruises in the Arctic Ocean under different sea-ice conditions in 2002, undisturbed benthic surface sediments were collected and assayed for the presence of a short-lived (t1/2 ¼ 53 d), particle-reactive cosmogenic radionuclide, 7Be, that is solely derived from atmospheric deposition. Under largely ice-covered conditions in May–June 2002, we did not detect this radionuclide in benthic surface sediments, despite significant inventories present in ice-rafted snow on the overlying sea ice (mean ¼ 86.8 Bq m2732.0 SD; n ¼ 9). During the July–August 2002 Shelf–Basin Interactions (SBI) cruise aboard the USCGC Healy and during a simultaneous cruise of the CCGS Sir Wilfrid Laurier on the Bering and Chukchi Shelf, which occupied the same general region following retreat and dissolution of Arctic ice cover, the 7Be present in this snow as well as surface deposition on to the sea ice-free water surface was detected in many benthic surface sediments, including some as deep as 945 m in Barrow Canyon. Inventories of 7Be in sediments were as high (60 Bq m2) as the entire decay-corrected inventory present earlier in some snow samples collected on the sea-ice cover. Other deposition indicators such as the inventories of sediment chlorophyll, sediment oxygen respiration rates and 234 Th-derived export fluxes also showed post-ice melt particle deposition and vertical transport, but in most cases the 7Be deposition was not tightly correlated with these other indicators, suggesting that 7Be sedimentation may not be controlled by the same processes. Our observations indicate that materials in sea ice, including contaminants, particulate organic, and mineral matter originating from atmospheric deposition or entrained in continental shelf sediments and rafted onto sea ice, can be rapidly transported to depth. The re-distribution of these materials as sea-ice drifts and eventually melts has the potential for impacting Arctic Ocean biogeochemical cycles and contaminant concentrations in areas of the Arctic remote from the original point of deposition. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sea ice; Beryllium-7; Cosmogenic isotopes; Rapid sedimentation; Arctic
1. Introduction
Corresponding author. Tel.: +1 865 974 2990; fax: +1 865 974 7896. E-mail address: [email protected] (L.W. Cooper).
0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.10.011
The exclusive natural source of the cosmogenic radionuclide 7Be is from production by cosmic ray spallation in the atmosphere (Arnold and Al-Salih, 1955). The radionuclide is deposited to the earth surface as an aerosol and with precipitation (Arnold
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and Al-Salih, 1955; Lal et al., 1958). In the few studies that have been accomplished in ice-covered seas, it has been expected that this radionuclide would be present in snow on sea-ice cover but not in the water column and underlying sediments in the absence of significant open-water (Grebmeier and Cooper, 1995; Kadko, 2000; Cooper et al., 2002; Eicken et al., 2002; Kadko and Swartz, 2004). In particular, Grebmeier and Cooper (1995) and Cooper et al. (2002) used 7Be as a marker for sedimentation within a polynya (open-water) and lead system in an otherwise ice-covered sea. In a similar way, the bomb fallout radionuclide 137Cs that is associated with clay particles in marine sediments was detected in portions of the Ross Sea where polynyas and other open water areas typically form each year but not under permanent ice cover (Grebmeier et al., 2003). In work we accomplished in 2002, as part of the Shelf–Basin Interactions (SBI) process study of Arctic shelf and basin exchange and also sampling associated with the Bering Strait Environmental Observatory on the Bering and Chukchi shelf, we used the receding ice pack in early summer as a presumed source of the radionuclide to underlying sediments. We expected that continuous sea-ice cover would prevent this atmospherically derived radionuclide from reaching benthic sediments prior to sea ice retreat. Because this radionuclide is comparatively short-lived (t1/2 ¼ 53 d), it would also provide an opportunity to observe seasonal deposition cycles as snow cover on ice melted and was introduced to the water column during sea-ice retreat and dissolution. Elemental beryllium has an affinity for particles and typical open-ocean profiles show preferential removal from surface waters (Measures and Edmond, 1982), so we expected that at least some 7Be would become attached to particles, whether in the melting snow, within sea ice, or in the surface water column, and be subsequently deposited to the sea floor. Our goal during two SBI process cruises was to document these short-term deposition events in conjunction with other indicators of recent particle deposition, such as changes in inventories of chlorophyll a in surface sediments (Cooper et al., 2002), sediment oxygen demand (Grebmeier and Cooper, 1995), as well as to compare the distribution of recently deposited 7Be to other tracers of particle export such as 234Th (Moran et al., 2005). It is worth pointing out that these particle export tracers and indicators do not necessarily record the same deposition processes as does 7Be. For example,
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sediment chlorophyll represents biologically based deposition from both sea-ice algae and watercolumn production, and sediment oxygen demand is a time-delayed response to recent biological deposition. The export flux of 234Th is also controlled by water-column processes (reviewed by Moran et al., 2003). By comparison, 7Be is best understood as a short-term tracer of particlereactive materials present within snow cover on the sea-ice surface, so our goal was as much to understand the differences as well as similarities to other sedimentation processes. 2. Methods Snow samples (0.0625 m2) were collected from the surface of sea ice during the early season research cruise (May–June 2002) prior to any significant melt (H. Eicken, pers. comm.). Care was taken to collect all snow present above the sea-ice surface. Snow was brought aboard ship, melted and shipped to the University of Tennessee for gamma spectroscopy. All counting was completed within 3 months of collection (2 half-lives); many samples were transported by helicopter to shore prior to the end of each cruise to facilitate faster initiation of gamma spectroscopy of the short-lived radionuclide. Prior to counting, liquid (snow) samples were concentrated by heating; weak acid (5% HCl) was used to keep 7Be in solution and prevent adherence to container walls. Precipitation samples collected during the summer (July–August 2002) cruise were handled similarly. Two precipitation collectors were mounted on the flying bridge of the USCGC Healy during the time that sediment sampling was underway, and precipitation flux inventories of 7Be during the cruise were determined by concentration and acid washing of the liquid precipitation samples as with the melted snow samples. These atmospheric flux data were extrapolated to a square meter basis, based upon the surface area of the precipitation collector, assuming steady-state deposition over the integrated collection period. Gamma spectroscopy of all samples was performed using a Canberra GR4020/S reverse electrode closed-end coaxial detector at the University of Tennessee. 7Be was detected by gamma decay at 477.6 KeV, using Canberra Genie 2000 software, which included corrections for efficiency and calibration with standards traceable to the US National Institute of Science and Technology. Surface sediments on all three cruises (two on the USCGC Healy and one on
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the CCGS Sir Wilfrid Laurier) were collected using either a single core or multi-core version of a 133cm2 HAPS corer (Kanneworff and Nicolaisen, 1973). Sediments were sectioned to depths of up to 15 cm, although no 7Be was detected below the 1-cm increment in any sample. Ninety cubic centimeters of each 1-cm sediment increment were added to aluminum cans with calibrated geometries for direct counting, using the same gamma spectroscopy system. When 7Be was not detected in surface sediments in the 90 cm3 cans, a larger 1-l sample of surface sediments from the same location, collected using a 0.10-m2 van Veen grab was counted, using 0.5- or 1-l Marinelli beakers with calibrated geometries. Since the van Veen grab penetrates to variable depths, and does not return entirely undisturbed surface sediments to the surface, it was not possible to accurately quantify the inventory of 7Be measured in these samples on a squaremeter basis, and we characterize these samples (not detectable in cores, but detectable in larger grab samples) as having ‘‘trace’’ inventories of 7Be. Despite the less quantifiable data obtained from the van Veen grab, use of this secondary sediment collection tool permits detection of the radionuclide in surface sediments at higher sensitivities because of the much larger volume of sample assayed. Samples for determination of chlorophyll a in surface (0–1 cm) sediments were also collected from the top 0–1 cm of the multi-HAPS cores. Following dark storage for 12 h in 90% acetone, chlorophyll a concentrations associated with small surface area aliquots of sediments were determined fluorometrically (Cooper et al., 2002). Other short-term
sedimentation data, including 234Th derived export fluxes and sediment oxygen respiration rates were drawn from collaborating studies also accomplished on the two USCGC Healy cruises (Moran et al., 2005; J. Grebmeier, unpublished data). 3. Results We determined the inventory of 7Be present in snow deposited on sea-ice cover over the Arctic shelf, slope and deep oceanic basin during the SBI Process cruise of the USCGC Healy in May–June 2002 (Table 1). In follow-up cruises of the Healy and the CCGS Sir Wilfrid Laurier in July–August, 2002, we determined the inventories of the radionuclide present in the top 1 cm of sediment cores collected in the same region following the retreat of sea ice (Table 2). As expected, because the continuous sea-ice cover present in the spring prevented the atmospherically derived radionuclide from reaching the benthic sediments prior to sea ice retreat, 7Be was not detectable in any benthic sediments sampled in May–June 2002, except for trace activity at Station HV1, immediately north of the Bering Strait, while the study area was covered with seasonal ice cover. The shipboard precipitation flux during the second cruise of the Healy, under open-water conditions, decay-corrected to 1 August 2002, was similar to the inventory present in snow on the sea ice during the first cruise (Table 1). When 7Be was detected in Marinelli beakers, but not in the 90-cm3 cans collected at the same location, these data are indicated as having had a trace of 7Be detected, as described in Section 2 (Table 2).
Table 1 Inventories of 7Be in surface snow and precipitation flux during second cruise of the USCGC Healy Location
Activity, 7Be (Bq m2)7SE at date of collection
Date of collection
Activity, 7Be (Bq m2) corrected to 1 August 2002
73.341N, 160.351W 73.741N, 158.941W 73.451N, 157.571W 73.101N, 158.211W 72.871N, 158.551W 72.611N, 158.771W 72.251N, 159.851W 71.491N, 153.901W 72.071N, 154.461W Precipitation flux Precipitation flux
94.278.0 58.874.4 99.470.4 119.5713.0 41.376.3 119.4711.2 81.277.0 122.377.0 45.277.9 32.572.4 35.472.9
20 May 2002 21 May 2002 23 May 2002 24 May 2002 27 May 2002 29 May 2002 30 May 2002 3 June 2002 5 June 2002 19 July–21 August 2002 19 July–21 August 2002
36.4 23.0 40.0 48.7 17.5 51.9 35.8 56.8 21.6 32.5 35.4
The decay-corrected values were calculated using ln(2)/(53)*difference (days) between the date of collection and a common date, 1 August 2002, that would permit comparison among samples collected on various dates.
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Table 2 Inventories of 7Be in surface (0–1 cm) sediments Station number, location
Date of collection
7
Be activity at date of collection (Bq m27SE)
Water depth (m)
Healy-12, 71.421N, 157.401W Healy-13, 71.641N, 155.901W Healy-14, 71.921N, 154.911W Healy-16, 72.181N, 153.911W Healy-17, 72.511N, 153.601W Healy-19, 71.961N, 152.131W Healy-20, 71.701N, 152.431W Healy-21, 71.641N, 152.381W Healy-22, 71.551N, 152.291W Healy-23, 71.441N, 152.461W Healy-24, 71.281N, 152.571W Healy-25, 72.231N, 159.331W Healy-26, 72.591N, 158.641W Healy-27, 72.711N, 158.521W Healy-35, 73.221N, 160.161W Healy-37, 73.051N, 160.421W Healy-38, 72.981N, 160.601W Healy-39, 72.731N, 161.321W Healy-41, 73.111N, 162.941W Healy-43, 73.621N, 165.471W Healy-44, 73.341N, 165.611W Healy-45, 70.661N, 166.561W Laurier-UTBS4, 64.961N, 169.881W Laurier-UTBS1, 64.991N, 169.141W Laurier-UTN1, 66.711N, 168.401W Laurier-UTN2, 67.051N, 168.731W Laurier-UTN3, 67.331N, 168.991W Laurier-UTN4, 67.501N, 168.911W Laurier-UTN5, 67.671N, 168.961W Laurier-UTN6, 67.741N, 168.441W Laurier-UTN7, 68.001N, 168.931W
21 July 2002 22 July 2002 23 July 2002 25 July 2002 27 July 2002 31 July 2002 1 August 2002 2 August 2002 3 August 2002 4 August 2002 4 August 2002 6 August 2002 7 August 2002 7 August 2002 16 August 2002 17 August 2002 18 August 2002 18 August 2002 19 August 2002 19 August 2002 20 August 2002 21 August 2002 17 July 2002 17 July 2002 17 July 2002 18 July 2002 18 July 2002 18 July 2002 18 July 2002 18 July 2002 18 July 2002
14.8711.7 18.875.6 17.776.5 n.d n.d. n.d. 12.9711.0 54.8710.8 Trace Trace 6.3717.6 16.276.0 Trace Trace n.d. Trace 15.776.3 60.2711.3 Trace 0.971.5 Trace 27.9750.3 31.8711.3 Trace 60.878.0 29.7734.7 Trace Trace n.d. Trace n.d.
127 162 563 1863 3197 2133 945 407 185 127 49 48 88 206 477 189 89 51 151 121 72 50 72 47 34 47 51 51 52 51 59
n.d. ¼ not detected; trace ¼ not detected in 90 cm3 aliquots collected using corer, but 7Be could be detected in 0.5-l Marinelli beakers collected using a 0.10 m2 van Veen grab, which were not quantifiable as to sediment surface area for the top 1 cm. A trace of 7Be was detected at the location of Laurier-UTN4 in May 2002, which was sampled from the USCGC Healy.
Decay-correction of the 7Be snow inventory in May–June 2002 to a common date of 1 August 2002 indicated that as much as 100% (60 Bq m2 as of 1 August 2002) of the 7Be inventory present in snow in May–June 2002 had reached surface sediments by the time of our sediment sampling in July–August 2002 (Table 2). This short-lived radionuclide was detected at depths as great as 945 m in Barrow Canyon (BC) to the north of Alaska, although it was also not detected in some shallower, inshore sediments (Fig. 1), showing an uneven pattern of particle deposition, with deposition apparently enhanced in portions of the Barrow submarine canyon (BC stations) and to the immediate east on the East Barrow (EB) transect line.
Other indicators of particle deposition, including sediment chlorophyll inventories, sediment oxygen respiration, and 234Th-derived export fluxes were evaluated in conjunction with these 7Be data. Sediment chlorophyll a inventories, for example, increased between the May–June and July–August 2002 cruises (Fig. 2). For the same stations occupied on both cruises, sediment chlorophyll a inventories on each cruise are correlated, although at the higher sediment chlorophyll a inventories (higher deposition), summer inventories fall well above a 1:1 ratio relative to spring inventories (Fig. 3), indicating enhanced deposition between the May–June 2002 and the July–August 2002 sampling.
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Fig. 1. 7Be fluxes from surface of sea ice to surface sediments of Arctic shelf and slope. Star symbols indicate inventory in snow on sea-ice cover, 20 May to 5 June 2002. Darker circles indicate an inventory was measurable on surface (0–1 cm) sediments. Lighter circles indicate trace activities present in surface sediments in Van Veen grab (1 l assayed), 21 July to 21 August 2002, but that the radionuclide was not detected in core tops (90 cm3 assayed) sampled at the same stations. White circles indicate locations where 7Be was not detected in core tops or grabs. White squares indicate locations where cores where collected, but samples were not assayed for 7Be. Gray tones shown reflect bathymetry, and where numbered, are depth (m).
234
Th-derived export fluxes and sediment oxygen respiration rates were also higher immediately before or during the July–August cruises (Moran
et al., 2005; J. Grebmeier, unpublished data) including enhanced deposition in BC and portions of the EB line where the highest 7Be fluxes also
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Fig. 2. Inventories of chlorophyll a (mg m2) present in surface (0–1 cm) sediments at tops of HAPS cores during three cruises in 2002. (A) May–June cruise of the USCGC Healy, (B) July–August cruises of the USCGC Healy and CCGS Sir Wilfrid Laurier.
were detected in surface sediments (Fig. 1; Table 2). Despite this, there were no statistically robust relationships between these alternate deposition indicators and the activities of 7Be that were detected in surface sediments on the July–August
2002 cruise (Fig. 4). In addition to the thoriumderived carbon fluxes (Fig. 4), we found no correlation between 7Be and the directly measured 234 Th flux data (Moran et al., 2004) from surface waters (data not shown).
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40
summer sediment chlorophyll a (mg m-2)
BC2 35
post-sea ice melt deposition 30
1:1 ratio (steady-state deposition)
WHS2
25
WHS1
20 15 10 5 0 0
5
10
15
20
25
spring sediment chlorophyll a (mg
30
35
40
m-2)
Fig. 3. Relationship between chlorophyll a in surface sediments collected on the May–June 2002 USCGC Healy cruise versus surface sediments collected at the same stations on the July–August 2002 cruises of the USCGC Healy and CCGS Sir Wilfrid Laurier.
4. Discussion We did not detect 7Be in any benthic sediments sampled in May–June 2002, except a trace activity in one station (HV1) immediately north of the Bering Strait, while the Arctic shelf, slope and deep basin was covered with seasonal ice cover. Consequently, virtually all 7Be observed in surface sediments in July–August 2002 was deposited during the time interval in summer before and during the second cruise as sea ice dissipated in the SBI study area. Stable oxygen isotope data indicate that freshwater contributed from melted sea ice was widespread in the study area by August 2002 replacing runoff that was almost exclusively the freshwater component source in May–June 2002 (Cooper et al., in press). Precipitation fluxes were similar during the July–August 2002 cruises as were existing inventories of 7Be in snow on surface sea ice, so it is not possible to rule out continuing deposition after sea ice melt from 7Be deposited to the sea surface in July–August 2002. However, other observations indicate that the earlier dissolution of sea ice was a key deposition precursor. For example, observations in May–June 2002 indicated that sea ice that was sampled in the Chukchi and Beaufort Seas carried significant sea-ice algal communities as well as ice-rafted sediments (H. Eicken, unpublished
data) that were entrained during sea-ice formation on continental shelves. It is possible that both sea-ice algae and ice-rafted sediments were the carrier mechanisms for rapid transport of the radionuclide deposited in snow to the sea floor. The inventory of chlorophyll a present in surface sediments increased between the spring and summer cruises (Fig. 2) but sea-ice algal communities were primarily present on the underside of the ice surface and may have been sedimented into the water column prior to coming in contact with 7Be deposited in melting snow on the sea-ice surface. The lack of any statistically robust relationships between the inventories of 7Be present at specific stations and sediment chlorophyll a, sediment oxygen respiration rates and 234Th-derived export fluxes also suggest that 7Be deposition may not be as strongly linked to sedimentation from marine phytoplankton production (water column or sea ice), but was rather tied to mineral-based sedimentation from sea ice. Although we were not able to explicitly test for mineral-based sedimentation, our rationale for this hypothesis is as follows. Most of the 7Be is associated with snow at the sea surface and most of the chlorophyll is associated with the ice-seawater interface on the underside of the sea ice. Since the sea ice carries ice-rafted sediments, and 7Be is known to be particle-reactive,
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sediment O2 respiration (mmol m-2 d-1)
sediment chlorophyll (0-1 cm; mg m-2)
40 35 30 25 20 15 10 5 0 40 35
10
20
30
40
50
60
70
10
20
30
40
50
60
70
0 7Be not 10 detected
20
30
40
50
60
70
30 25 20 15 10 5
234
Th-derived POC flux (mmol m-2 d-1)
50 40 30 20 10
80
7Be (Bq m-2)
Fig. 4. Inventories of 7Be in surface sediments (0–1 cm) collected on the July–August 2002 cruises of the USCGC Healy and CCGS Sir Wilfrid Laurier relative to samples collected at the same stations: (A) sediment chlorophyll (0–1 cm); (B) sediment oxygen respiration rates measured in shipboard incubations; (C) particle export, as estimated through 234Th and particulate organic carbon measurements using in-situ pumping methodologies. Additional details on sampling and methods for sediment oxygen respiration and 234Th assays are available elsewhere (Cooper et al., 2002; Moran et al., 2005).
it makes sense that the 7Be would come into contact with sediments in the ice prior to ice algae on the underside of sea ice. Consequently, it seems most likely that 7Be deposited in snow on the sea-ice surface infiltrated into channels as the surface snow cover and sea ice melted in early June 2002 (H. Eicken, pers. comm.), and the radionuclide became bound to ice-rafted sediments when in contact with sediments. Following further ice dissolution, the 7Be-bound sediments were then rapidly deposited to the ocean floor. It is worth noting that another study of 7Be sedimentation from multi-year ice during the SHEBA program indicates that a significant fraction of the ice-rafted radioisotope inventory can be
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trapped in melt ponds and not reach the upper mixed layer of the ocean (Eicken et al., 2002). However, in our study that was undertaken in much thinner seasonal ice, the sea ice present in May–June 2002 had essentially melted in place by July–August 2002, which is confirmed by the oxygen isotope composition of the surface ocean layers during the later sampling (Cooper et al., in press). Consequently, unlike at SHEBA, no ice was left for the 7 Be inventory to infiltrate into, which set up the highly efficient transfer of the radioisotope to the sea floor that we observed. Our examination of the linkage between 7Be deposition and other indicators of deposition from biological production (i.e., sediment chlorophyll a, sediment oxygen respiration rates and 234Th-derived export fluxes) should not be considered conclusive, however. Several factors limited our analyses, and it is noteworthy that the highest 7Be inventories were observed in regions such as BC and the slope portions of the EB transects (Table 2) where there were also indications of high biologically based sedimentation (Fig. 3; see also Moran et al., 2005; J. Grebmeier, unpublished data). Since these and other SBI studies are showing evidence of the importance of BC and ‘‘downstream’’ continental slope portions of the EB transects for deposition of organic materials, the same particle focusing processes may also influence the settling of sea ice rafted sediments. This also might explain why some of the higher 7Be activities were observed in relatively deep water and not necessarily on the shallow shelves where vertical transport distances from the sea surface are shorter. Our data analyses were limited by relatively high counting errors for 7 Be (Table 2) and the number of locations where no 7 Be was detected, but where indicators such as sediment chlorophyll a could be reproducibly measured (e.g., Figs. 3 and 4). This suggests that relatively low sensitivity for 7Be detection as well as the magnitude of counting errors were in part responsible for the low degree of correlation between 7Be inventories and other deposition indicators. Chlorophyll a also has a much longer biological half-life in sediments (Itakura et al., 1997; Lewis et al., 1999); we observed that it was present in sediments during the May–June 2002 cruise when there were no indications of high contemporaneous sedimentation. 234Th-derived export flux data also were limited in this study to the water column (Moran et al., 2005). Although both 234Th and 7 Be are gamma emitters, self-absorption of the
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energetically weak gamma-ray associated with 234Th decay in the calibrated containers we used that were necessary to sensitively assay 7Be prevented us from obtaining data on excess (not supported) 234Th present in surface sediments. Because of the short half-lives of both 234Th and 7Be (24 and 53 d, respectively), significant additional radioisotope counting instrumentation will have to be available to conclusively determine the differences in the relationships among 234Th and 7Be inventories in sediments and marine sedimentation processes. Our results do provide indications that particlereactive material, including contaminants such as heavy metals, organic materials, and radionuclides present in sea ice cover are likely to become bound to larger particles as sea ice melts and will be quickly deposited to the sea floor. These findings have consequences for biogeochemical cycles and food chains in Arctic systems. The benthos on various Arctic shelves are considered particularly important as food sources for benthic-feeding whales, walrus, bearded seals, and diving ducks (Grebmeier et al., 1995). There is, moreover, evidence for an increasingly seasonal pattern of sea-ice cover in the Arctic as the more permanent ice retreats (e.g., McPhee et al., 1998; Rothrock et al., 1999). Thus, contaminants deposited on Arctic sea ice will be more quickly incorporated into surface sediments and potentially into food webs. Particularly in areas of the Arctic where sea ice melts, such as Fram Strait, higher degrees of deposition of sea ice-rafted materials, including contaminants, is to be expected (Pfirman et al., 1995; Cooper et al., 1998; Masque´ et al., 2003). These findings also support the hypothesis (Cooper et al., 2000) that deposition from sea-ice cover is the mechanism responsible for sedimentation of plutonium with isotopic ratios consistent with near-field tropospheric, rather than global stratospheric bomb fallout in much of the deep Arctic Ocean. The Arctic Ocean has very low inventories of plutonium present in deep-sea Arctic sediments, but somewhat paradoxically, over much of the deep Arctic basin, the proportion of the fissionable isotope 239Pu is comparatively high (i.e., plutonium isotope ratios of 240Pu/239Pu are low), indicating that the origin of the plutonium is not simply stratospheric bomb fallout as is the case in most of the rest of the world (Cooper et al., 2000; Smith et al., 2000) The same rapid deposition mechanism from the atmosphere to the benthos that we observed here for 7Be also could have
played a role in the rapid deposition of tropospheric bomb fallout from Arctic nuclear tests such as on Novaya Zemlya tests during the late 1950s to early 1960s. While other sources are possible, such as reprocessed nuclear fuel cycle waste, regional distribution of near-field, low-yield tropospheric bomb fallout on Arctic sea ice might have been the source of the low 240Pu/239Pu ratio material that is found in deep Arctic Ocean sediments. Sedimentation from sea ice could have led to the widespread distribution of plutonium with comparative low ratios of 240Pu/239Pu ratios in much of the deep Arctic Ocean.
5. Conclusions Sampling of benthic surface sediments in May– June 2002 and July–August 2002 in shelf and slope regions of the Arctic Ocean indicate that a high proportion, up to 100%, of the cosmogenic radionuclide 7Be present on the sea-ice surface prior to snowmelt and sea-ice dissolution, is deposited to benthic sediments within 3 months. Deposition was detected to depths as great as 945 m, and the geographical patterns of distribution showed that deposition was highest in shelf regions, and in some deeper waters in BC. Nevertheless, deposition of 7Be was not conclusively linked to other deposition indicators of biological production, including 234Th export, 234Th-derived carbon export fluxes, sediment chlorophyll a, and sediment oxygen uptake, suggesting that the deposition may be primarily driven by mineralogical attachment of 7Be to sea ice-rafted sediments.
Acknowledgements We thank Hajo Eicken and Rolf Gradinger for collecting the snow samples, and Jim Bartlett, Betty Carvellas, Jacyln Clement, Pat Kelly, Rick Nelson, Sandor Mulsow and the marine science technician team aboard the USCGC Healy and the deck crew aboard the CCGS Sir Wilfrid Laurier for field assistance in collecting samples. Two anonymous reviewers, Hajo Eicken and editor Rodger Harvey provided comments that helped improve earlier versions of the manuscript. Financial support was provided to the Shelf-Basin Interactions and Bering Strait Environmental Observatory projects by the US National Science Foundation.
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