Fiona A. McLaughlin

A Major Ecosystem Shift in the Northern Bering Sea

Until recently, northern Bering Sea ecosystems were characterized by extensive seasonal sea ice cover, high water column and sediment carbon production, and tight pelagic-benthic coupling of organic production. Here, we show that these ecosystems are shifting away from these characteristics. Changes in biological communities are contemporaneous with shifts in regional atmospheric and hydrographic forcing. In the past decade, geographic displacement of marine mammal population distributions has coincided with a reduction of benthic prey populations, an increase in pelagic fish, a reduction in sea ice, and an increase in air and ocean temperatures. These changes now observed on the shallow shelf of the northern Bering Sea should be expected to affect a much broader portion of the Pacific-influenced sector of the Arctic Ocean.

Smallest Algae Thrive As the Arctic Ocean Freshens

In the Arctic Ocean, phytoplankton cell sizes have decreased with warming temperatures and fresher surface waters. As climate changes and the upper Arctic Ocean receives more heat and fresh water, it becomes more difficult for mixing processes to deliver nutrients from depth to the surface for phytoplankton growth. Competitive advantage will presumably accrue to small cells because they are more effective in acquiring nutrients and less susceptible to gravitational settling than large cells. Since 2004, we have discerned an increase in the smallest algae and bacteria along with a concomitant decrease in somewhat larger algae. If this trend toward a community of smaller cells is sustained, it may lead to reduced biological production at higher trophic levels.

Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean

Received 27 December 2005; revised 7 March 2006; accepted 13 March 2006; published 21 April 2006. [1] The spatial pattern of recent ice reduction in the Arctic Ocean is similar to the distribution of warm Pacific Summer Water (PSW) that interflows the upper portion of halocline in the southern Canada Basin. Increases in PSW temperature in the basin are also well-correlated with the onset of sea-ice reduction that began in the late 1990s. However, increases in PSW temperature in the basin do not correlate with the temperature of upstream source water in the northeastern Bering Sea, suggesting that there is another mechanism which controls these concurrent changes in ice cover and upper ocean temperature. We propose a feedback mechanism whereby the delayed sea-ice formation in early winter, which began in 1997/1998, reduced internal ice stresses and thus allowed a more efficient coupling of anticyclonic wind forcing to the upper ocean. This, in turn, increased the flux of warm PSW into the basin and caused the catastrophic changes. Citation: Shimada, K., T. Kamoshida, M. Itoh, S. Nishino, E. Carmack, F. A. McLaughlin, S. Zimmermann, and A. Proshutinsky (2006), Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean, Geophys. Res. Lett., 33, L08605,

Beaufort Gyre freshwater reservoir: State and variability from observations

[1] We investigate basin-scale mechanisms regulating anomalies in freshwater content (FWC) in the Beaufort Gyre (BG) of the Arctic Ocean using historical observations and data collected in 2003–2007. Specifically, the mean annual cycle and interannual and decadal FWC variability are explored. The major cause of the large FWC in the BG is the process of Ekman pumping (EP) due to the Arctic High anticyclonic circulation centered in the BG. The mean seasonal cycle of liquid FWC is a result of interplay between the mechanical (EP) and thermal (ice transformations) factors and has two peaks. One peak occurs around June–July when the sea ice thickness reaches its minimum (maximum ice melt). The second maximum is observed in November–January when wind curl is strongest (maximum EP) and the salt input from the growing ice has not yet reached its maximum. Interannual changes in FWC during 2003–2007 are characterized by a strong positive trend in the region varying by location with a maximum of approximately 170 cm a � 1 in the center of EP influenced region. Decadal FWC variability in the period 1950–2000 is dominated by a significant change in the 1990s forced by an atmospheric circulation regime change. The center of maximum FWC shifted to the southeast and appeared to contract in area relative to the pre-1990s climatology. In spite of the areal reduction, the spatially integrated FWC increased by over 1000 km 3 relative to climatology.

Physical and geochemical properties across the Atlantic/Pacific water mass front in the southern Canadian Basin

Temperature, salinity, nutrients, oxygen, and halocarbon data collected in the Arctic Ocean reveal a frontal structure previously unrecognized in the hydrography of the Canadian Basin. Samples were collected on a 1300-km section extending from the Beaufort Sea in the Canada Basin to the East Siberian Sea in the Makarov Basin. These data, collected in 1993 aboard the CCGS Henry Larsen, reveal a lateral boundary between water masses of Atlantic and Pacific origin. The term “water mass assembly” is introduced to describe the basic arrangement or vertical stacking of water masses found in the Arctic Ocean, recognizing that water mass components within each assembly may differ from basin to basin. Using historical data, two primary water mass assemblies are defined, each consisting of three layers: an upper layer, an Atlantic layer, and a deep layer. These two assemblies are marked by important differences. One assembly, here defined as the Western Arctic (WA) assembly, is characterized by an upper layer of relatively fresh, high-nutrient water of Pacific origin; below this, by an Atlantic layer with a core temperature generally below 0.5°C; and, finally, by a deep layer of higher salinities and colder temperatures (about −0.5°C) than found in the overlying Atlantic layer. The second assembly, here defined as Eastern Arctic (EA) assembly, is characterized by the absence of Pacific water in the upper layer; below this, by an Atlantic layer core as warm as 2° to 3°C; and by a colder (about −0.9°C) deep layer. Because the presence or absence of Pacific origin water is a key characteristic distinguishing the two assemblies, we will refer to the water mass boundary between the two assemblies as the Atlantic/Pacific front. Earlier research indicated that water masses in the Arctic Ocean were separated by a front above the Lomonosov Ridge into the Canadian and Eurasian basins. Although all Larsen-93 stations from the Canada Basin (A1–D1) display classic WA assembly characteristics, the Makarov Basin station (E1) shows EA assembly characteristics in the upper and Atlantic layers and a WA assembly deep layer. This suggests a relocation in the position of the Atlantic/Pacific boundary away from the Lomonosov Ridge. Further, Larsen-93 data show the transition region between the Atlantic and deep layers is fresher in the Makarov Basin than corresponding water in either the Canada or Eurasian basins, implying a source of cold, low-salinity water, perhaps from the Laptev and East Siberian shelves. The front separating these two assemblies lies above the Mendeleyev Ridge and is marked by large lateral gradients in all measured properties. In particular, the penetration of anthropogenic halocarbons is 2 to 3 times deeper in the Makarov Basin than in the Canada Basin, implying enhanced rates of ventilation. This suggests that direct exchange between the Canadian and Eurasian basins has occurred recently near the perimeter and that physical and chemical properties, including contaminants, may have been transported by boundary currents more quickly from one basin to the other.

Aragonite Undersaturation in the Arctic Ocean: Effects of Ocean Acidification and Sea Ice Melt

Acidic Ocean One consequence of the historically unprecedented level of CO2 in the atmosphere that fossil fuel burning has caused, in addition to a warmer climate, is higher concentrations of dissolved CO2 in the oceans. This dissolved CO2 makes the oceans more acidic, and thus less saturated with respect to calcium carbonate. This has important ramifications for organisms that have calcium carbonate skeletons, which depend for their survival on the saturation state of calcium carbonate in the waters where they live. Yamamoto-Kawai et al. (p. 1098) report that in 2008, surface waters of the Canada Basin became undersaturated with respect to aragonite, a relatively soluble form of calcium carbonate incorporated into the shells or skeletons of many types of marine plankton and invertebrate. This undersaturation occurred much sooner than had been anticipated and has important implications for the composition of the Arctic ecosystem. Surface waters in the Canada Basin were undersaturated with respect to aragonite in 2008, earlier than predicted. The increase in anthropogenic carbon dioxide emissions and attendant increase in ocean acidification and sea ice melt act together to decrease the saturation state of calcium carbonate in the Canada Basin of the Arctic Ocean. In 2008, surface waters were undersaturated with respect to aragonite, a relatively soluble form of calcium carbonate found in plankton and invertebrates. Undersaturation was found to be a direct consequence of the recent extensive melting of sea ice in the Canada Basin. In addition, the retreat of the ice edge well past the shelf-break has produced conditions favorable to enhanced upwelling of subsurface, aragonite-undersaturated water onto the Arctic continental shelf. Undersaturation will affect both planktonic and benthic calcifying biota and therefore the composition of the Arctic ecosystem.

Waters of the Makarov and Canada basins

Abstract Hydrographic measurements from the 1994 Arctic Ocean Section show how the Makarov and Canada basins of the Arctic Ocean are related, and demonstrate their oceanographic connections to the Eurasian Basin. The inflow into the Makarov Basin consists largely of well-ventilated water within a broad band of densities from a boundary flow over the Siberian end of the Lomonosov Ridge. The boundary flow contains a significant component of dense shelf water likely originating in the Barents, Kara, and Laptev Seas. Earlier ice camp data show that the Canada Basin is relatively more isolated from this ventilation source. In the Canada Basin shelf sources influenced by Bering Sea water appear to add cold waters with high silicate concentrations to the halocline and deeper. In 1994 the halocline silicate maximum over the central Makarov Basin was absent, evidence of the recent displacement of the upper (S∼ 33.1) halocline water from the Chukchi-East Siberian Sea region by water from the Eurasian Basin. Much of the Makarov Basin water in and below the halocline is in fact from the Eurasian Basin, with admixture of waters from the Canada Basin suggested by their higher silicate concentrations. Mid-depth eddies may transport anomalous properties into the central Arctic and create property gradients or fronts in mid-depth and deep waters. The complex topography of the Mendeleyev Ridge-Chukchi Plateau region also may assist spreading of water from the boundary into the interior. Atlantic layer characteristics in 1994 differed from previous general depictions. In particular the core temperatures at the Chukchi-Mendeleyev boundary were at least 0.2°C warmer on average than indicated in earlier work. The recent warming at intermediate depth has resulted from inflow of Atlantic waters that have been cooled relatively little during their transit of the Norwegian Sea.

Evidence for warming of Atlantic water in the Southern Canadian Basin of the Arctic Ocean: Results from the Larsen‐93 Expedition

Potential temperature (θ) and salinity (S) data obtained along the perimeter of the southern Canadian Basin north of the East Siberian Sea in 1993 aboard the CCGS Henry Larsen show higher temperatures in waters of Atlantic origin than in available climatological data for the Canadian Basin. In particular, a front is observed near the Mendeleyev Ridge which separates the cooler Atlantic waters of the Canada Basin from the warmer Atlantic waters observed in the Makarov Basin. The front is further characterized by a change in the θ/S slope of Arctic thermocline water, and by thermohaline intrusions (θ and S reversals) within the Atlantic layer. The idea that this warm variety of Atlantic water has come recently from the Eurasian Basin is supported by its higher level of the tracer CFC-11.

Changes in temperature and tracer distributions within the Arctic Ocean: results from the 1994 Arctic Ocean section

Abstract Major changes in temperature and tracer properties within the Arctic Ocean are evident in a comparison of data obtained during the 1994 Arctic Ocean Section to earlier measurements. (1) Anomalously warm and well-ventilated waters are now found in the Nansen, Amundsen and Makarov basins, with the largest temperature differences, as much as 1 °C, in the core of the Atlantic layer (200–400 m). Thus thermohaline transition appears to follow from two distinct mechanisms: narrow (order 100 km), topographically-steered cyclonic flows that rapidly carry new water around the perimeters of the basins; and multiple intrusions, 40–60 m thick, which extend laterally into the basin interiors. (2) Altered nutrient distributions that within the halocline distinguish water masses of Pacific and Atlantic origins likewise point to a basin-wide redistribution of properties. (3) Distributions of CFCs associated with inflows from adjacent shelf regions and from the Atlantic demonstrate recent ventilation to depths exceeding 1800 m. (4) Concentrations of the pesticide HCH in the surface and halocline layers are supersaturated with respect to present atmospheric concentrations and show that the ice-capped Arctic Ocean is now a source to the global atmosphere of this contaminant. (5) The radionuclide 129I is now widespread throughout the Arctic Ocean. Although the current level of 129I level poses no significant radiological threat, its rapid arrival and wide distribution illustrate the speed and extent to which waterborne contaminants are dispersed within the Arctic Ocean on pathways along which other contaminants can travel from western European or Russian sources.

Alkane, terpene and polycyclic aromatic hydrocarbon geochemistry of the Mackenzie River and Mackenzie shelf: Riverine contributions to Beaufort Sea coastal sediment

Abstract To study the largest source of river sediment to the Arctic Ocean, we have collected suspended particulates from the Mackenzie River in all seasons and sediments from the Mackenzie shelf between the river mouth and the shelf edge. These samples have been analyzed for alkanes, triterpenes and polycyclic aromatic hydrocarbons (PAHs). We found that naturally occurring hydrocarbons predominate in the river and on the shelf. These hydrocarbons include biogenic alkanes and triterpenes with a higher plant/peat origin, diagenetic PAHs from peat and plant detritus, petrogenic alkanes, triterpenes and PAHs from oil seeps and/or bitumens and combustion PAHs that are likely relict in peat deposits. Because these components vary independently, the season is found to strongly influence the concentration and composition of hydrocarbons in the Mackenzie River. While essentially the same pattern of alkanes, diagenetic hopanes and alkyl PAHs is observed in all river and most shelf sediment samples, alkane and triterpene concentration variations are strongly linked to the relative amount of higher plant/peat material. Polycyclic aromatic hydrocarbon molecular-mass profiles also appear to be tied primarily to varying proportions of peat, with an additional petrogenic component which is most likely associated with lithic material mobilized by the Mackenzie River at freshet. Consistent with the general lack of alkyl PAHs in peat, the higher PAHs found in the river are probably derived from forest and tundra fires. A few anthropogenic/pyrogenic compounds are manifest only at the shelf edge, probably due to a weakening of the river influence. We take this observation of pyrogenic PAHs and the pronounced source differences between two sediment samples collected at the shelf edge as evidence of a transition from dominance by the Mackenzie River to the geochemistry prevalent in Arctic regions far removed from major rivers.