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Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 55 56 57
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OF
, Alfonso Mucci
a
a, b
, Bjørn Sundby
a, c
d
, Aiguo Gao ,
Department of Earth and Planetary Sciences, McGill University, 3450 University, Montreal, QC H3A 2A7, Canada ` Montre´al, C.P. 8888, Succ. Centre-Ville, Montre´al, QC H3C 3P8, Canada GEOTOP McGill-UQAM Research Centre, Universite´ du Que´bec A c ` Rimouski, 310 alle´e des Ursulines, C.P. 3300, Rimouski, QC G5L 3A1, Canada Institut des Sciences de la Mer de Rimouski, Universite´ du Que´bec a d College of Oceanography and Environmental Science, Department of Oceanography, Xiamen University, 182 University Road, Xiamen, Fujian 361005, China e Ishinomaki-Senshu University, Shinmito Minamisakai Ishinomaki-shi MIYAGI. 986-8580, Japan b
RO
12 13
, Sean A. Crowe
a, 2
DP
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Ce´dric Magen , Gwe´nae¨lle Chaillou Ryosuke Makabe e, 3, Hiroshi Sasaki e
a,1
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2009 Accepted 10 September 2009 Available online xxx
To establish the relative importance of terrigenous and marine organic matter in the southern Beaufort Sea, we measured the concentrations and the stable isotopic compositions of organic carbon and total nitrogen in sediments and in settling particles intercepted by sediment traps. The organic carbon content of surface sediment in the Chukchi and southern Beaufort Seas ranged from 0.6 to 1.6% dry wt., without a clear geographical pattern. The CORG:NTOT ratio ranged from 7.0 to 10.4 and did not vary significantly downcore at any one station. Values of d13CORG and d15NTOT in the sediment samples were strongly correlated, with the highest values, indicative of a more marine contribution, in the Amundsen Gulf. In contrast, the organic matter content, elemental (CORG:NTOT ratio) and isotopic (d13CORG and d15NTOT) composition of the settling particles was different from and much more variable than in the bottom sediments. The isotopic signature (d13CORG and d15NTOT) of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a labile marine component produced in situ by planktonic organisms, a refractory marine component, the end product of respiration and diagenesis, and a refractory terrigenous component. A three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates an apparent two-component mixing model of the organic matter in sediments. The suspended matter in the water column contains organic matter varying from essentially labile and marine to mostly refractory and terrigenous. As it settles through the water column, the labile marine organic matter is degraded, and its original stable isotope signature changes towards the signature of the marine refractory component. This process continues in the bottom sediment with the result that the sedimentary organic matter becomes dominated by the refractory terrigenous and marine components. Ó 2009 Published by Elsevier Ltd.
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a, b, *
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Keywords: organic matter carbon nitrogen sediment stable isotopes Arctic ocean Beaufort Sea Amundsen gulf Mackenzie Shelf
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Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf region, Canadian Arctic
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* Corresponding author. Present address: Department of Oceanography, Florida State University, 117 N. Woodward Avenue, Oceanography/Statistics building, Tallahassee, FL 36306-4320, USA. E-mail addresses:
[email protected] (C. Magen), gwenaelle_chaillou@uqar. qc.ca (G. Chaillou),
[email protected] (S.A. Crowe),
[email protected] (A. Mucci),
[email protected] (B. Sundby), aggao@fio.org.cn (A. Gao),
[email protected] (H. Makabe),
[email protected] (H. Sasaki). 1 Departement de Biologie, Chimie et Ge´ographie, Universite´ du Que´bec a` Rimouski, Rimouski, Canada. 2 Nordic Center for Earth Evolution, University of Southern Denmark, Odense, Denmark. 3 National Institute of Polar Research, Tokyo, Japan.
60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94
1. Introduction The changing climate in the Arctic is decreasing the extent and thickness of the ice cover, increasing the length of the ice-free season in the Arctic Ocean, and modifying the production rate and fluxes of marine organic carbon (Arrigo et al., 2008; Comiso et al., 2008; Haas et al., 2008; Boe et al., 2009; Lavoie et al., 2009; Overland, 2009). The changing climate is also altering the timing and magnitude of river discharge and coastal erosion, and the delivery of terrigenous organic carbon to the Arctic Ocean (Peterson et al., 2002; De´ry et al., 2009; Jones et al., 2009; Spencer et al., 2009). In consequence, the relative importance of the fluxes of terrestrial and marine organic carbon to the seafloor will likely change, as will the
0272-7714/$ – see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.ecss.2009.09.009
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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processing and preservation of organic carbon in Arctic sediments (Katsev et al., 2006). Shallow continental shelf regions account for about 50% of the total surface area of the Arctic Ocean, which sets the Arctic apart from other oceans (Jakobsson et al., 2004), and allows a much larger proportion of the particulate organic matter that settles through the water column to reach the seafloor without being significantly degraded (Jahnke, 1996). Outside of the Mackenzie River plume and occasional nepheloid layers, most of the particulate organic carbon encountered in the water column of the Mackenzie Shelf is marine ˜ i et al., 2005; Forest et al., 2007), even though the Mackenzie (Gon River contributes vast quantities of terrigenous organic carbon to the Beaufort Sea (Rachold et al., 2004). Despite the predominantly marine provenance of the suspended organic carbon, the shelf sediments appear to be dominated by terrigenous organic carbon ˜ i et al., 2005). The organic carbon that is ultimately preserved (Gon
CO
143 144
Stations
167
CA10 CA11 CA04
168 169
CA07
170 171 172 173 174 175
197 198 199 200 201
in Beaufort Sea sediments may thus bear little resemblance to the material settling through the water column. The primary objective of our study was to evaluate the influence of the Mackenzie River on the composition, distribution, and reactivity of sedimentary organic carbon in the southern Beaufort Sea – Amundsen Gulf region. To this end, we traced the origin and fate of terrigenous and marine organic matter in the water column and sediment on the basis of its elemental (CORG:NTOT ratio) and stable isotope (d13CORG and d15NTOT) composition. We also provide a reference state relative to which modifications resulting from climatic change can be evaluated.
CA05 CA15 CA08 CA18
Depth (m)
Sampling period
105 102 109 214 94 200 105 108 212 108 109 413
07 06 04 04 04 04 13 11 11 13 14 14
Oct 2003–08 Sep 2004 Oct 2003–08 Sep 2004 Aug 2003–07 Sep 2004 Oct 2003–04 Aug 2004 Aug 2003–07 Sep 2004 Oct 2003–04 Aug 2004 Oct 2003–28 Jul 2004 Oct 2003–22 Jul 2004 Oct 2003–22 Jul 2004 Oct 2003–18 Jul 2004 Oct 2003–29 Jul 2004 Oct 2003–29 Jul 2004
202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219
2. Study area The study area includes the Mackenzie Shelf, the adjacent continental slope to a depth of w1000 m, and the Amundsen Gulf (Fig. 1).
220 221 222 223 224 225
Table 1 Sediment trap data obtained during the 2003–2004 sampling year. All results are averages over the sampling period.
163 164 165 166
195 196
Fig. 1. Map of the CASES survey area. Stations 101, 109, 500 and 718 were sampled during Leg 2 (Oct–Nov 2003). All other stations were sampled during Leg 8 (Jun–Aug 2004). The black filled circles correspond to stations where sediment cores were retrieved. The four smaller black dots in the upper left inset correspond to stations sampled in the Chukchi Sea during the First (stations C9 and C29 – Aug 1999) and Second (stations R6 and M1 – Jul 2003) Chinese National Arctic Research Expeditions. The empty triangles refer to stations were sediment traps were deployed.
UN
141 142
193 194
ED
135 136
139 140
191 192
PR
130 131
137 138
189 190
OO
124 125
Suspended particulate matter (mg m2 d1)
CORG (mg m2 d1)
CORG (%)
NTOT (mg m2 d1)
NTOT (%)
d13CORG
d15(NTOT)
(&)
&
CORG:NTOT (molar ratio)
718 323 104 182 62 74 147 81 58 129 73 144
19.0 12.7 7.7 4.8 7.6 3.3 10.8 6.2 3.0 9.2 8.6 3.9
2.6 4.0 7.5 3.8 12.6 4.1 8.4 7.6 6.0 7.4 12.5 2.4
2.3 1.7 1.3 0.5 1.2 0.4 1.4 1.0 0.5 1.5 1.6 0.5
0.3 0.5 1.2 0.4 1.9 0.5 1.1 1.2 1.0 1.2 2.3 0.3
26.5 26.8 26.0 25.5 26.8 25.7 26.3 25.6 25.5 25.3 24.5 25.3
5.8 6.9 10.3 5.7 10.1 6.5 15.8 14.8 10.7 13.8 11.9 6.7
9.5 8.9 7.1 10.1 7.7 9.3 8.9 7.1 7.3 7.1 6.4 9.5
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
226 227 228 229 230 231 232 233 234 235 236 237 238 239 240
246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263
Stations Mackenzie shelf 709 711 718 906 912 Mackenzie Slope 600 750 850 Amundsen Gulf 101 109 409 500 Chukchi Sea C29 C9 R6 M1
d13CORG
d15NTOT
(&)
(&)
83 74 42 271 56
0.5 0.6 0.5 0.4 0.7
0.9 1.4 1.6 1.6 1.5
9.0 8.8 9.7 10.1 10.4
24.7 24.8 25.7 25.2 25.9
5.1 5.9 4.5 4.7 3.5
71.66 71.34 70.55
584 1096 1075
0.5 0.4 0.3
1.4 1.4 1.5
8.7 9.3 9.0
24.7 24.4 24.7
6.1 5.5 5.4
121.28 123.43 127.09 127.58
70.60 70.66 71.51 72.06
458 570 378 389
1.5 1.3 0.5 1.4
1.2 1.2 1.7 1.4
9.0 9.2 8.3 9.0
23.4 23.5 23.5 23.6
7.8 7.9 7.8 7.2
321 322
165.03 175.03 169.00 169.01
73.45 70.50 69.50 77.30
92 54 53 1456
0.1 0.1 0.2 1.8
1.9 2.0 1.2 0.6
8.4 8.6 9.1 7.0
22.1 22.2 23.2 22.3
9.0 8.4 8.2 7.8
325 326
133.78 133.80 133.55 138.60 137.94
70.95 70.80 70.18 70.20 69.49
130.66 134.18 137.60
279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305
311 312 313 314 315 316 317 318 319 320
323 324
327 328 329 330 331
Chukchi Sea was used in this study as a reference coastal environment dominated by marine organic matter.
332 333
3. Material and methods
334 335
3.1. Sampling
336 337
ED
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With a discharge of 3.3 1011 m3 yr1, the Mackenzie River is the main source of freshwater to the Beaufort Sea. It also provides 99% of the total land-derived suspended particulate matter brought into the Canadian Arctic Ocean (Rachold et al., 2004). The amounts of terrigenous particulate and dissolved organic carbon transported to the shelf are estimated at 1.3 and 2.1 Mt a1, respectively (Macdonald et al., 1998). Marine mud, predominantly silt and clay-sized, covers most of the Mackenzie Shelf (Hill et al.,1991). After the ice break-up in late May, freshwater from the Mackenzie River forms a 5-m thick buoyant plume that spreads over the more saline shelf water. This plume is characterized by a high suspended particulate load whose concentration decreases offshore. Algae bound to ice contribute 10–15% of the annual primary production (O’Brien et al., 2006). The freshwater and suspended particulate matter inputs to the Amundsen Gulf are likely much smaller than to the Mackenzie Shelf as no large rivers discharge into the Gulf. This large basin exceeds 600 m in depth in places (station 109 – Fig. 1) and is >300 m deep over most of its surface. To date, there is little information regarding the sedimentary regime in this area. Surface sediments in the Amundsen Gulf are composed of silts (Bader and Henry, 1958), with a fluid upper layer overlying more compacted deposits. The most recent surveys reveal that Holocene sedimentation is minimal and that the seafloor mostly consists of glacial marine deposits and exposed bedrock (S. Blasco, pers. comm.). Satellite data suggest that the Cape Bathurst polynya, located to the north-west, may contribute significantly to the flux of organic carbon settling to the deep Amundsen Gulf (Arrigo and van Dijken, 2004). High nutrient concentrations in the deep waters of the Amundsen Gulf are interpreted as resulting from the export and remineralization of organic matter from the surface to the deep waters (Simpson et al., 2008). The relatively shallow Chukchi Sea (10–60 m) is bounded by the East Siberian Sea to the west and the Alaskan Shelf to the east, and is separated by the Bering Strait from the Bering Sea to the south. It is a transition zone where the nutrient-rich Pacific water enters the Arctic Ocean (Macdonald et al., 2004a), causing its waters to be highly productive. Although terrestrial material from the East Siberian Sea and Yukon River contributes to the sedimentary carbon pool of the Chukchi Sea, marine sedimentary organic matter becomes largely dominant offshore (Belicka et al., 2002, 2004). The
UN
271 272
309 310
CORG:NTOT (molar ratio)
Depth (m)
265 266
269 270
307 308
CORG (% dry wt.)
Latitude ( )
264
267 268
306
CINORG (% dry wt.)
Longitude ( )
F
244 245
3
Table 2 Sediment data obtained during the 2003–2004 sampling year. All data provided for the Chukchi Sea are from the 0–2 cm sediment layer. For the Mackenzie Shelf and Slope as well as the Amundsen Gulf, the data correspond to the 0 – 0.5 cm sediment layer, with the exception of station 709 (0.5 – 1.0 cm). The CORG:NTOT ratios are not corrected for NBOUND.
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Sediment traps were deployed from early October 2003 to late August 2004 at several depths in the Mackenzie Trough, on the Mackenzie Slope, and in the Amundsen Gulf (Table 1). Each mooring line was equipped with a Nichiyu Giken Kogyo SMD26S-6000 conical, automated sediment trap (0.5 m2 aperture, 26-cup turntable, sampling periods of 1–29 days). Formalin was added to each cup to preserve the collected material (5% v/v, sodium borate buffered). The presence of formalin typically depletes the d13CORG signature of the trap material by less than 1.65& and does not significantly affect the d15N values (Arrington and Winemiller, 2002; Sarakinos et al., 2002). Nevertheless, upon recovery, the samples were thoroughly rinsed with seawater that was previously filtered on pre-combusted 0.7 mm WhatmanÒ glass fiber filters. Zooplankton greater than 5 mm and swimmers were removed from the samples with a 1 mm sieve and by handpicking under a stereomicroscope. A total of 12 undisturbed sediment cores were recovered on the Mackenzie Shelf and Slope, and in the Amundsen Gulf (Fig. 1) using a 0.12 m2 Ocean Instruments Mark II box corer. The 30–50 cm long cores were sub-sampled into horizontal layers (i.e., 0.5 cm intervals at the surface, and up to 5 cm intervals at depth) in a glove-box purged by a continuous flow of nitrogen to minimize sediment oxidation (Edenborn et al., 1986). Solid samples were transferred to pre-weighed scintillation vials and stored at 20 C on-board ship. Upon return to McGill University, the vials were weighed, freezedried, and re-weighed to determine the water content. The sediment porosity was calculated using a dry sediment density of 2.65 g cm3 (Berner, 1980) and the bottom water salinity. The freeze-dried sediments were then homogenized by grinding in an agate mortar for subsequent solid phase analyses. Samples from the Chukchi Sea were retrieved during the first and second Chinese National Arctic Research Expeditions either with a single box corer or a multi-tube corer. The top 10 cm of the
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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core was sub-sampled at 1 cm intervals. The remainder of the core was sliced at 2 cm intervals. Samples were frozen on-board at 20 C for later analyses. 3.2. Elemental and stable isotopic composition of the organic matter Aliquots of sediment trap material were filtered through preweighed WhatmanÒ glass fiber filters combusted for 4 h at 450 C. The filters were dried for 12 h at 60 C, exposed for 12 h to concentrated HCl fumes to remove inorganic carbon, and finally analyzed for organic carbon with a Perkin Elmer CHNS 2600 Series II elemental analyzer. Although some studies have shown that acidification could alter the d13C and d15N values of the suspended organic matter (e.g. Froelich, 1980; Bunn et al., 1995), it was recently shown that drying the filters at 60 C and exposing them to HCl fumes does not induce a significant change (Lorrain et al., 2003). The total carbon and nitrogen content of the freeze–dried and homogenized sediments were determined using a Carlo-ErbaÔ NC
453 454 455 456 457 458 459 460 461 462 463 464 465 466 467
470 471 472 473 474 475 476 477
Fig. 2. Total inorganic and organic carbon concentrations at all sampled stations.
CO
415 416
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413 414
451 452
468 469
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407 408
449 450
2500 elemental analyzer. The absolute instrumental reproducibility of these analyses, as determined from replicate measurements of Organic Analytical Standard substances (Acetanilide, Atropine, Cyclohexanone-2,4-Dinitrophenyl-Hydrazone and Urea), was estimated at 0.1% for CORG and 0.3% for NTOT and the relative analytical reproducibility was 5%. The total inorganic carbon (CINORG) content was analyzed independently on distinct aliquots of the freeze–dried sediments using a UIC Coulometrics coulometer following acidification of the samples and CO2 extraction. The analytical reproducibility was better than 5%. The total organic carbon (CORG) content was obtained by subtracting CINORG from CTOT. Aliquots of sediment from the surface, intermediate and bottom depth intervals of each core (typically the 0–0.5 cm, 5–7 cm and 27–30 cm intervals) were washed twice for 24 h with a dilute HCl (1 M) solution to dissolve the solid carbonates. The acid supernatant was decanted and solids rinsed with distilled water, decanted, dried and ground before the stable isotopic composition (d13CORG and d15NTOT) of the organic matter was measured with a CarloErbaÔ elemental analyzer in line with a GV Instruments IsoPrimeÔ
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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mass spectrometer. Data are reported in the standard notation in & with respect to V-PDB for carbon (Coplen, 1995) and atmospheric N2 for nitrogen. The analytical uncertainty was 0.1& for carbon and 0.2& for nitrogen. The international standard IAEA-C6 sucrose was measured several times during the isotopic analyses of carbon and yielded an average value of 10.8 0.1& (n ¼ 4) whereas the accepted value is 10.8 0.5&. The stable isotope composition of the organic carbon and total nitrogen (d13CORG and d15NTOT) in aliquots of the sediment trap material were analyzed with a Europa PPZ GEO 20-20 mass spectrometer. The analytical errors for these nitrogen and carbon isotopic analyses were 0.1& and 0.3&, respectively. Studies have shown that rinsing sediment with acid could alter the d13CORG and d15NTOT values (Bunn et al., 1995; Schubert and Nielsen, 2000). However, in a recent study carried out on sediments, Kennedy et al. (2005) showed that acidification with a weak acid (1–2 M HCl) is the most appropriate method to remove inorganic carbon without altering the stable isotope signature of the organic matter.
583 584 585 586 587 588 589 590 591 592 593 594 595 596 597
600 601 602 603 604 605 606 607 608 609
Fig. 3. Vertical profiles of the CORG:NTOT molar ratios in sediment cores recovered within the study area. Note the different scale for station M1.
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4. Results 4.1. Organic and inorganic carbon content of sediment traps and sediment The settling flux of particulate matter at w100 m water depth, averaged over the sampling period, was highest at stations located near the mouth of the Mackenzie River (CA10 and CA11– Fig. 1, Table 1). The average CORG content (weight ratio) of the settling particulate matter was lowest in the Mackenzie Trough (2.6% at CA10) and highest in the Mackenzie Slope and Amundsen Gulf (12.6% at CA07 and 12.5% at CA18). The average NTOT content (weight ratio) ranged between 0.3% in the Mackenzie Trough (CA10) and 2.3% in the Amundsen Gulf (CA18). With the exception of station 709 on the Mackenzie Shelf (0.9%) and the Chukchi Sea (0.6 to 2.0%), sedimentary CORG concentrations in core tops all fell within a narrow range (1.2–1.6%) (Table 2) and decreased slightly with depth in most cores (Fig. 2). In contrast to
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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CORG, large spatial variations in CINORG content were observed in surface sediments: the highest concentrations were found in the Amundsen Gulf (0.5–1.5% dry wt.) and at station M1 in the Chukchi Sea (1.8% dry wt.) whereas the lowest concentrations were observed at stations C9, C29 and R6 in the Chukchi Sea (<0.2% dry wt.) and on the Mackenzie Shelf and Slope (<0.5% dry wt.) (Table 2, Fig. 2). 4.2. CORG:NTOT ratio and stable isotope composition of organic C and N The CORG:NTOT molar ratio of the sediment trap material collected at w100 m depth was highest in the Mackenzie Trough (9.5 at CA10) and lowest in the Amundsen Gulf (6.4 at CA18) (Table 1). Where two sediments traps were deployed at the same station (i.e. CA04, CA07, CA15 and CA18) but at different depths (i.e. w100 and 200 or 400 m – Table 1), the CORG:NTOT ratio was lowest in the
713 714 715 716 717 718 719 720 721 722 723 724 725 726 727
730 731 732 733 734 735 736 737 738 739 740 741
Fig. 4. Vertical profiles of the d13CORG and d15NTOT values in sediment cores recovered within the study area.
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material retrieved at the shallower depth (w100 m). Surface sediment CORG:NTOT values ranged from 7.0 at station M1 to 10.4 at station 912 (Table 2), but showed no significant downcore variation (Fig. 3). The d13CORG signature of the sediment trap material ranged from 26.8& at CA07 and CA11, to 24.5& at CA18. There was a slight increase (þ1&) of the d13CORG with water depth at stations CA07 and CA18 (Table 1). The d13CORG values of surface sediments were highest in the Chukchi Sea (23.2& to 22.1&), >24.2& in the Amundsen Gulf, intermediate (24.7& to 24.4&) on the Mackenzie Slope and <24.7& on the Mackenzie Shelf (Table 2, Fig. 4). The d15NTOT values of the settling particulate matter varied spatially, with the lowest values being observed on the Mackenzie Slope (þ5.7& at CA10) and the highest ones in the Amundsen Gulf (þ15.8& at CA05) (Table 1). In contrast to d13CORG, the d15NTOT values always decreased with increasing water depth. In surface
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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Fig. 5. Stable isotope composition of the organic matter recovered from sediment traps and sediment cores in the sampling area. The surface samples were taken from the 0–0.5 cm interval (except for station 709 – 0.5–1.0 cm), the intermediate depth samples from the 5–7 cm interval and the deep samples were from the 27–30 cm interval of the cores. Samples for the Chukchi Sea sediments were obtained from the First and Second Chinese National Arctic Research Expeditions. In this case, the intermediate depth corresponds to the following sampling intervals: 10–12 cm at R6, 8–9 cm at C9 and C29, and 14–16 cm at M1 whereas the deep sediments were sampled from the 30–32 cm interval at M1 and the 32– 34 cm interval at C9. 15
sediments, the d NTOT values decreased from þ9.0& in the Chukchi Sea to þ3.5& on the Mackenzie Shelf (Table 2, Fig. 4).
RR E
791 792
5. Discussion
801 802
Terrestrial organic matter is produced by vascular plants and freshwater plankton, to which erosion adds soil organic matter of varying age. Marine organic matter, principally of autotrophic and heterotrophic origin, is an equally complex mixture of compounds. Both terrestrial and marine organic carbon contain materials of
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different origin, age, and reactivity that can be altered during transport and deposition (Eglinton et al., 1997). The relative contribution of terrigenous and marine organic matter to the suspended particulate and sedimentary organic carbon pools has often been determined by measuring the elemental CORG:NTOT ratio and the stable isotope composition (d13CORG and d15NTOT) of the organic matter. These measurements can give reliable estimates of the relative contributions of the sources if no more than two components with distinct and well defined CORG:NTOT ratios or d13CORG or d15NTOT signatures contribute to the bulk organic matter (Thornton
839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881
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Fig. 6. Ternary diagram showing the contribution of each organic matter end-member to the bulk organic matter. The diagram shows the settling (sediment trap) organic matter recovered at 100 m (black filled symbols), at 200 or 400 m (gray filled symbols) and the sedimentary organic matter (empty symbols). Note that some stations could not be included in this diagram due to our choice of end-members.
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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Fig. 7. Percentages of terrigenous, refractory marine and fresh marine CORG in the bulk organic matter of surface sediments collected throughout the sampling area, calculated using both d13CORG and d15NTOT.
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5.1. The CORG:NTOT ratio in Beaufort Sea sediments
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5.2. The CORG:NTOT ratio in Beaufort Sea settling suspended matter
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929 930
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947
In recent studies of Arctic Ocean sediments (e.g. Schubert and Calvert, 2001; Stein and Macdonald, 2004), the CORG:NTOT ratio was corrected to CORG:NORG to account for the abundance of inorganic nitrogen. In the Beaufort and Chukchi Seas however, bound inorganic nitrogen (NBOUND) appears to be a minor fraction of the total nitrogen, and the CORG:NTOT ratios should not be significantly different from the CORG:NORG ratios (Macdonald et al., 2004a; Naidu et al., 2004). Our estimates of NBOUND, determined by the method described by Schubert and Calvert (2001), confirm that inorganic nitrogen does not affect the CORG:NORG ratio in Beaufort Sea sediments. The CORG:NTOT ratios in surface sediments of the Mackenzie Shelf and Slope (Table 2) varied within a narrow range of 8.7–10.4, in ˜ i et al. (2000) and Naidu et al. agreement with values reported by Gon (2000). The highest ratios were found at the two stations (906 and 912) closest to the mouth of the Mackenzie River. Except for the latter two, the CORG:NTOT ratios in Shelf and Slope sediments are not different from values found in the surface sediments of the Amundsen Gulf (Table 2). Naidu et al. (2000) also found that the CORG:NTOT ratios in Kugmallit Bay and the Mackenzie River delta sediments were not significantly different from those of the Mackenzie Shelf. These results are consistent with the conclusion of Macdonald et al. (2004b) that the sedimentary CORG:NTOT ratio is not a suitable tracer of the origin of the sedimentary organic matter in the Beaufort and Chukchi Seas.
925 926
with lower sediment fluxes and higher carbon contents. At about 100 m below the sea surface, the contribution of terrigenous organic matter to the settling particulate matter flux is more important on the Mackenzie Trough than on the Mackenzie Slope or in the Amundsen Gulf. The high CORG:NTOT ratios reflect the influence of the Mackenzie River, where ratios as high as 17.6 (O’Brien et al., 2006) and 22.1 (Mucci et al., 2008) have been observed in suspended particulate matter near the outflow of the Mackenzie River. Emmerton et al. (2008) found similarly high ratios (17 < CORG:N < 20) in the suspended particulate matter of the Mackenzie River.
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and McManus,1994). For instance, the CORG:NORG ratios in the Laptev Sea (10–25) and the Kara Sea (10–15) are clearly indicative of a terrigenous origin (Stein and Fahl, 2004a,b). Nevertheless, the simple two-component model may not always be appropriate, as we will show below to be the case for the Beaufort Sea, and may have to be replaced by more complex models.
CT
915 916
PR
908 909
The CORG:NTOT molar ratios measured on sediment trap material from stations CA10 and CA11 in the Mackenzie Trough (Table 1) varied within somewhat broader limits (6.4–10.8) than the surface sediments, consistent with previous data obtained in the same area (O’Brien et al., 2006). The lowest CORG:NTOT ratios generally coincided
5.3. Comparing the stable isotope composition of settling particulate and sedimentary organic matter When d13CORG is plotted against d15NTOT for all available sediment core samples, the result is a fairly tight linear relationship that holds for all sampling depths (Fig. 5b, c, d). One possible interpretation is that the organic matter accumulating in the sediments is a mixture of only two types of materials, each with its own distinctive isotopic signature. However, this simple relationship does not hold for the suspended matter intercepted by the sediment traps: none of the isotope data for the suspended particulate matter (SPM) plots with the sediment isotope data (Fig. 5a). A large scatter in the d13CORG vs d15NTOT relationship of the SPM was also observed by O’Brien et al. (2006), who attributed it to local and seasonal variations in the contributions of the different sources of organic matter, i.e. freshwater plankton, marine plankton and terrigenous organic matter. Clearly, while a two-component mixing model could fit the sediment data reasonably well, it cannot accommodate the sediment trap data. Instead, we propose that a three-component mixing model with fresh, easily degradable marine organic matter as one component, and refractory terrestrial and refractory marine organic matter as the two other components is more appropriate for the Beaufort Sea.
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5.4. A three-component mixing model for organic matter
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The choice of d13CORG value for the terrigenous end-member (27&) for the three-component model is based on the isotopic composition of material recovered from the Mackenzie and Colville River deltas as well as coastal peat samples (Macdonald et al.,
1017 1018
Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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aþbþc ¼ 1
(3)
15
15
where d13C and d15N are the isotopic signatures of the terrigenous (TERR; 27.0& and þ1.0&, respectively), fresh marine (MARf; 25.5& and þ16.5&, respectively) and refractory (degraded) marine (MARr; 21.2& and þ9.0&, respectively) organic matter end-members. The coefficients a, b and c are the fractions of terrigenous, fresh marine and refractory marine organic matter, respectively. A ternary diagram was then constructed in which the contributions of each end-member to the bulk organic matter for all sediment trap and most sediment samples are represented (Fig. 6). The same calculation was applied to the sediment trap and sediment data reported in O’Brien et al. (2006). The ternary diagram retains the linearity of the d13CORG – d15NTOT relationship displayed by the sediment data and highlights the more refractory nature of the sedimentary organic matter: all the sediment data fall on a line that is close to and nearly parallel to the line that connects the refractory terrestrial and marine endmembers. The diagram also shows that the settling organic matter (sediment trap material) contains more, sometimes much more, of the reactive marine component than the sediments. The material intercepted by the deeper traps appears to contain a smaller proportion of the reactive marine component than the material trapped closer to the sea surface, indicating that organic matter is being degraded as it settles through the water column. Nevertheless, the trend is not without exceptions and these may reflect the capture of resuspended sediment transported offshore from the shallow shelf environment to the shallower (w100 m) traps. In the Mackenzie Shelf region, resuspension of sediment from the Shelf to the Slope is well documented (Carmack and Macdonald, 2002; O’Brien et al., 2006; Forest et al., 2007) and can be triggered by storm events (O’Brien et al., 2006; Mucci et al., 2008) and shelf-break upwelling (Williams and Carmack, 2008). These resuspension events could explain the observed increase in total particulate flux with depth at stations CA04, CA07 and CA18 (Table 1). A nearly 130% increase of the total particulate flux between 100 and 400 m at CA18 (Table 1) suggests that sediment resuspension also takes place in the Amundsen Gulf, consistent with in situ transmissivity measurements carried out in this area (Mucci et al., 2008). The nearly depth-invariant organic carbon content and isotopic composition in the sediment cores recovered over our study area may seem surprising, but it likely reflects the loss of the most reactive components during settling through the water column and early diagenesis within the sediment. It also reflects the fact that the mass flux of settling material to the seafloor is small compared to the mass of refractory organic matter contained in the sediment reservoir to which this material is being added and mixed into. For these reasons, the sedimentary organic matter within the sampling area is dominated by the least reactive components of marine and terrestrial organic matter, i.e. by refractory organic marine and terrestrial organic carbon, as shown in Fig. 7. It should be noted that this comparison is based on the composition of the solid phase only, and does not preclude temporal modifications of the redox regime in the sediment pore water caused by pulsed delivery of reactive organic carbon to the seafloor (Gobeil et al., 2001; Katsev et al., 2006). Using the particulate CORG:aluminum ratio (POC:Al) of O’Brien et al. (2006), we calculated the contribution of terrigenous and marine organic matter to the settling organic matter intercepted by sediment traps set at w100 m depth (Fig. 8). The calculations are consistent with what we obtained from the d13CORG and d15NTOT
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1998; Naidu et al., 2000). This value is consistent with the d13CORG composition of sediment trap material recovered in the Mackenzie Trough during this study (Fig. 5a) as well as with the isotopic composition of bulk suspended sediment collected by centrifugation in late spring – early summer in three channels of the ˜ i et al., 2005). It differs from the lower Mackenzie River (Gon d13CORG (i.e. <31&) reported by Retamal et al. (2007) for seston sampled in the Mackenzie River delta, who invoked a contribution by freshwater plankton. Our choice for the terrigenous endmember d15NTOT value (þ1&) is based on the analysis of fluvial/ deltaic sediment (Naidu et al., 2000) and suspended sediment samples recovered in one channel of the Mackenzie River estuary (Retamal et al., 2007) and near the coast in the Kugmallit Trough (O’Brien et al., 2006). The d13CORG and d15NTOT signature of the fresh, marine organic matter is difficult to pin down because the taxonomic composition of phyto- and zoo-plankton populations varies seasonally and according to nutrient availability. This is especially true in the Arctic where it switches abruptly from dominantly ice-bound (ice algae) to pelagic populations in the spring. Schell et al. (1998) measured the stable isotope signature of three zooplankton-groups in the Bering, Chukchi and Beaufort Seas. In the Canadian and eastern Alaskan Beaufort Sea, they reported low d13CORG (25.7& to 23.4&) and high d15NTOT (þ9.2& to 13.5&) values for all three groups. Iken et al. (2005) reported similar results for zooplankton species at stations located in the Amundsen Gulf and on the Mackenzie Slope. The d15NTOT values are consistent with our measurements of sediment trap material recovered at w100 m in the Amundsen Gulf and Mackenzie Slope (d15NTOT ¼ þ10.1& to þ15.8& – Table 1, Fig. 5a). Given the large uncertainties, we chose 25.5& for d13CORG and þ16.5& for d15NTOT for the fresh, marine organic matter end-member as these values allowed most our data to be encompassed within a ternary diagram. The fresh organic matter end-member corresponds to the organic matter found in 100 m sediment traps. Finally, for the refractory marine end-member, we chose data from Chukchi Sea sediments, where the organic matter is dominated by a marine source. Naidu et al. (1993) reported a d13CORG of 21.2&, and our own measurements gave a d15NTOT of þ9& (Fig. 5b). The contribution of each organic matter end-member to the bulk material recovered in the sediment traps and sediment can then be estimated using the following set of equations:
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Fig. 8. Percentages of terrigenous and marine CORG in the settling material collected in sediment traps throughout the sampling area, calculated using POCterr:Al ¼ 0.16 (see O’Brien et al. (2006)).
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(2)
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ad NTERR þ bd NMARf þ cd NMARr ¼ d NBULK
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(1)
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ad CTERR þ bd CMARf þ cd CMARr ¼ d CBULK
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signatures (Fig. 6 – the marine organic matter was dominant in the Mackenzie Slope and Amundsen Gulf), but lead to higher values for the contributions of the marine organic matter. Given the variability in the stable isotope composition of seston in the Arctic Ocean, this discrepancy is likely related to the choice of the fresh marine end-member. The POC:Al ratios provide a good estimate of the terrigenous contribution from the Mackenzie River to the suspended organic matter pool because Al is an excellent tracer of terrigenous material. However, it cannot be used to differentiate between multiple marine end-members.
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The isotopic signature (d13CORG and d15NTOT) of organic matter in the Beaufort Sea is well constrained by three distinct end-members: a refractory terrigenous component, a labile marine component produced in situ by planktonic organisms, and a refractory marine component – the end product of respiration and diagenesis. The three-component mixing model explains the scatter observed in the stable isotope signatures of the sediment trap samples and accommodates the apparent two-component mixing of the organic matter in bottom sediments. The relative proportion of each of the three end-members can be estimated by plotting the stable isotope data on a ternary diagram. The diagram reveals that the suspended matter in the water column contains organic matter varying from essentially labile and marine organic matter, as in the Amundsen Gulf, to mostly refractory and terrigenous organic matter, as in the Mackenzie Trough. As the suspended particulate matter settles through the water column, the labile marine organic matter is degraded and its original stable isotope signature changes towards the signature of the refractory components. This process continues in the sediment with the result that the sedimentary organic matter is dominated by the refractory terrigenous and marine components. The relatively constant d13CORG and d15NTOT values with core depth also show that, irrespective of the isotopic composition of fresh organic matter originally produced in the water column, the refractory organic matter that remains after respiration and early diagenesis has a nearly invariant isotopic signature. Although the isotope data indicate an almost exclusively refractory sedimentary organic matter, redox chemistry profiles for this area show that the organic matter in these sediments is nevertheless being oxidized (Gobeil et al., 2001; Mucci et al., 2008). The proportion of terrigenous organic matter in the sediments on the Mackenzie Shelf decreases with increasing distance from the Mackenzie River delta, whereas little or no terrigenous organic matter has accumulated in Amundsen Gulf sediments. Because the refractory components of the organic matter that originally settled to the seafloor dominate the composition of the sedimentary organic matter of this region, the CORG/NTOT ratio of the organic matter is not a sensitive measure of the relative contributions of terrestrial and marine organic matter sources to the Beaufort Sea sediments.
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Altabet, 1988; Altabet and Francois, 1994; Druffel et al., 1998; Holmes et al., 1996; Nakatsuka et al., 1997; Schafer and Ittekkot, 1993; Tremblay et al., 2008; Wu et al., 1999.
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6. Conclusions
We thank Louis Fortier and his associates, Martin Fortier, Luc Michaud and Marc Ringuette, for their leadership during the Canadian Arctic Shelf Exchange Study (CASES) and the captains and the crew of the CCGS Amundsen for their assistance at sea. We are also grateful to two anonymous reviewers who helped improve this
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manuscript. This study was funded by Natural Sciences and Engineering Research Council of Canada (NSERC) CASES Network and Discovery grants to A. M. and B. S. We thank Dr. J.-F. He´lie of the GEOTOP-UQAM-McGill Research Center for the elemental and isotopic analyses. GEOTOP is funded through a grant from the Fonds Que´becois de la Recherche sur la Nature et les Technologies (FQRNT). Financial support was also provided in the form of stipends or scholarships to C. M. from the Department of Earth and Planetary Sciences at McGill University, NSERC and GEOTOP.
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Please cite this article in press as: Magen, C., et al., Origin and fate of particulate organic matter in the southern Beaufort Sea – Amundsen gulf..., Estuar. Coast. Shelf Sci. (2009), doi:10.1016/j.ecss.2009.09.009
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