River water and wrine inventory over the Laptev Sea shelf : 2007 to 2011

OS51C-1005 Five years of oxygen isotope and hydrological survey reveal interannual variations in the inventory and distribution of river water over the Laptev Sea. Our results suggest that the Arctic Dipole Anomaly might connect the Laptev Sea river water inventory and the global Arctic freshwater inventory. In 2007, 2009 and 2010 relatively low amount of river water (≤1500 km3) was found and was mostly located in the southeastern Laptev Sea. In 2008 and 2011, high amounts of river water (~1600 km3 and ~2000 km3) were found, especially in the central and northern part of the shelf, suggesting a northward export of this water. It has been suggested that atmospheric forcing mainly controls the Laptev Sea summer surface hydrography and for this period, the interannual variability or summer river water inventory is coherent with the summer Arctic Dipole index. This could suggest that the Arctic Dipole has been a dominant forcing controlling the distribution and the fate of river water discharged within the Laptev Sea over the 2007-2011 period, which is concurrent with the recently highlighted persistent shift in early summer Arctic atmospheric circulation (Overland et al., 2012, GRL 39, L19804). The variation in river water inventory over the Laptev Sea Shelf is also positively related with recent Arctic Basin and Beaufort Gyre freshwater inventory (with a 2-yrs lag), which suggest that the river water originating from the Laptev Sea have an impact on the global Arctic freshwater inventory. During the same period the brine inventory was also variable but was dissociated from the river water inventory variation suggesting that, during this period different forcing was influencing the brine inventory

Oxygen isotope composition of living Neogloboquadrina pachyderma (sin.) in the Arctic Ocean

Data from the Nansen Basin of the Arctic Ocean are used to investigate the habitat and conditions under which the polar planktic foraminifer Neogloboquadrina pachyderma (sin.) calcifies. The vertical distribution of δ18O values of net-sampled speciments, together with their abundances and proportion of calcification, are compared with δ18O values from both water samples and foraminiferal tests from core-top sediments. Within the Nansen Basin the average depth of habitat of N. pachyderma (sin.) changes from about 150 m in the southern part to about 80 m in the northern. The average depth of calcification, however, in both regimes varies between 100 and 200 m water depth. δ18O data from net sampled N. pachyderma (sin.) are directly reflected in the core-top sediment data, but compared to equilibrium calcite δ18O values derived from measurements of the ambient water, a consistent offset of about 1‰ over all depth intervals is observed. While in the southern part of the Nansen Basin advection through Fram Strait of planktic foraminifers from further south may play a role, the data from the northern part of the Nansen Basin give clear evidence that the observed offset in δ18O values is caused by a vital effect of N. pachyderma (sin.)

Shelf basin exchange along the Siberian continental margin: modification of Atlantic Water and Lower Halocline Water

Highlights • Atlantic Water modified by sea-ice melt and meteoric water at Barents Sea slope • LHW may be divided into different types by Principal Component Analysis (PCA) • high salinity LHW-type forms in the Barents and Kara seas • low salinity LHW-types form in the western Laptev Sea or enter via Vilkitsky Strait • PCA does not support a distinction between onshore and offshore LHW branches Abstract Salinity and stable oxygen isotope (δ18O) evidence shows a modification of Atlantic Water in the Arctic Ocean by a mixture of sea-ice meltwater and meteoric waters along the Barents Sea continental margin. On average no further influence of meteoric waters is detectable within the core of the Atlantic Water east of the Kara Sea as indicated by constant δ18O, while salinity further decreases along the Siberian continental slope. Lower halocline waters (LHW) may be divided into different types by Principal Component Analysis. All LHW types show the addition of river water and an influence of sea-ice formation to a varying extent. The geographical distribution of LHW types suggest that the high salinity type of LHW forms in the Barents and Kara seas, while other LHW types are formed either in the northwestern Laptev Sea or from southeastern Kara Sea waters that enter the northwestern Laptev Sea through Vilkitsky Strait. No further modification of LHW is seen in the eastern Laptev Sea but the distribution of LHW-types suggest a bifurcation of LHW at this location, possibly with one branch continuing along the continental margin and a second branch along the Lomonosov Ridge. We see no pronounced distinction between onshore and offshore LHW types, as the LHW components that are found within the halocline over the basin also show a narrow bottom-bound distribution at the continental slope that is consistent with a shelf boundary current as well as a jet of water entering the western Laptev Sea from the Kara Sea through Vilkitsky Strait

Stable oxygen and carbon isotopes in modern benthic foraminifera from the Laptev Sea shelf: implications for reconstructing proglacial and profluvial environments in the Arctic

Measurements of δ18O and δ13C isotopes in three benthic foraminiferal species from surface sediments of the eastern Laptev Sea are compared to water δ18O values and δ13C values of dissolved inorganic carbon (DIC). Samples investigated originate from two environmentally contrasting core locations, which are influenced by riverine freshwater runoff to a varying degree. At the river-distal site, located within relatively stable marine conditions on the outer shelf, Elphidiella groenlandica, Haynesina orbiculare and Elphidium excavatum forma clavata show a positive specific offset of 1.4‰, 1.5‰ and 1‰, respectively, in their δ18O values relative to the expected value for inorganic calcite precipitated under equilibrium conditions. At the site close to the Lena River confluence, with enhanced seasonal hydrographic contrasts, calculated δ18O offsets in E. groenlandica and in H. orbiculare remain about the same whereas E. e. clavata displays a distinctly negative offset of −1.8‰. The δ18O variation in E. e. clavata is interpreted as a vital effect, a finding which limits the potential of this species for reconstructing freshwater-influenced shelf paleoenvironments on the basis of oxygen isotopes. This interpretation gains support when comparing foraminiferal δ13C with the δ13CDIC of the water. While some of the difference in the carbonate δ13C seems to be controlled by a riverine-related admixture of DIC, clearly defined δ13C ranges in each of the three foraminifera at the river-proximal site shows that also the carbon isotopic signature in E. e. clavata is particularly affected by environmental factors

Freshwater balance and the sources of deep and bottom waters in the Arctic Ocean inferred from the distribution of H218O

Data from sections across the Eurasian Basin of the Arctic Ocean occupied in 1987 and 1991 are used to derive information on the freshwater balance of the Arctic Ocean and on sources of the deep waters of the Nansen, Amundsen and Makarov basins. Using salinity, H218O, and mass balances we estimate the river-runoff and the sea-ice melt water fractions contained in the upper waters of the Arctic Ocean and infer pathways of the river-runoff signal from the shelf seas across the central Arctic Ocean to Fram Strait. The average mean residence time of the river-runoff fraction contained in the Arctic Ocean halocline is determined to be about 11 to 14 years. Pacific water entering through Bering Strait is traced using silicate and its influence on the halocline waters of the Canadian Basin is estimated. Water column inventories of river-runoff and sea-ice melt water are calculated for a section just north of Fram Strait and implications of these inventories for sea-ice export through Fram Strait are discussed. Comparison of the ratios of shelf water, Atlantic water and the deep waters of the Arctic Ocean indicate that the sources of the deep and bottom waters of the Eurasian Basin are located in the Barents and Kara seas

The impact of climatic and atmospheric teleconnections on the brine inventory over the Laptev Sea shelf between 2007 and 2011

Export of brine-enriched water from Siberian shelves is thought to be a key parameter in maintaining the Arctic Halocline, which isolates the fresh and cold surface water from the warm Atlantic water and thus prevent dramatic change in the Arctic sea-ice thermodynamic. In this study, we used five years of oxygen isotope and hydrological summer surveys to better understand the factors controlling the brine inventory and distribution over the Laptev Sea shelf. The inventory was maximal in 2011 and 2007 and minimal in 2010. The brine inventory interannual variations are coherent with the winter Arctic Oscillation index that was maximal in 2011 and 2007 and minimal in 2010, which is known to modulate Arctic winds and sea-ice export pattern. While we should remain cautious since our record is limited to 5-years, our results suggest that the combined effect of the Arctic Oscillation and of the Arctic Dipole is the main factor controlling the annual variations in the inventory of brine-enriched waters from the Laptev Sea shelf between 2007 and 2011, especially during extreme negative Arctic Oscillation and Arctic Dipole conditions as in 2010

Arctic river-runoff: mean residence time on the shelves and in the halocline

The mean residence time of river-runoff on the shelves and in the halocline of the Arctic Ocean is estimated from salinity and tracer data (tritium, 3He and the 18O/16O ratio). These estimates are derived from comparison of apparent tracer ages of the halocline waters using a combination of tracers that yield different information: (1) the tritium “vintage” age, which records the time that has passed since the river-runoff entered the shelf; and (2) the tritium/3He age, which reflects the time since the shelf waters left the shelf. The difference between the ages determined by these two methods is about 3–6 years. Correction for the initial tritium/3He age of the shelf waters (about 0.5–1.5 years) yields a mean residence time of the river-runoff on the shelves of the Siberian Seas of about 3.5 ± 2 years

The imprint of anthropogenic CO2 in the Arctic Ocean: evidence from planktic δ13C data from watercolumn and sediment surfaces

δ13C values of N. pachyderma (sin.) from the water column and from core top sediments are compared in order to determine the 13C decrease caused by the addition of anthropogenic CO2 to the atmosphere. This effect, which is referred to as the surface ocean Suess effect, is estimated to be about −0.9‰(±0.2‰) within the Arctic Ocean halocline waters and to about −0.6‰(±0.1‰) in the Atlantic-derived waters of the southern Nansen Basin. This means that the area where the Arctic Ocean halocline waters are formed, the Arctic shelf regions, are relatively well ventilated with respect to CO2. Nevertheless, δ13C of dissolved inorganic carbon (δ13CDIC) in the Arctic Ocean halocline waters is far from isotopic equilibrium. Absolute values of δ13C of N. pachyderma (sin.) covary with the surface ocean Suess effect, and we interprete changes in both parameters as a reflection of the degree of ventilation of the waters on the shelf sea. Measurements of δ13C of N. pachyderma (sin.) in the Arctic Ocean from plankton tows reveal a “vital effect” of about −2‰, significantly different from other published values. A first-order estimate of the total anthropogenic carbon inventory shows, that despite of its permanent sea-ice cover, the Arctic Ocean, with 2% of the global ocean area, is responsible for about 4–6% of the global ocean's CO2 uptake

Arctic waters on the verge of changing?

Satellite images document a decrease in Arctic ice cover in summer by 40% during the past 30 years. However, responsible processes and possible consequences of this decrease are little understood. The joint Russian-German project „Laptev Sea Polynya“ is coordinated by IFM-GEOMAR and aims to ascertain the causes and consequences of climate change and its essential mechanisms in the Arctic. Now the project scientists revealed a changed water mass distribution in the Siberian Laptev Sea which might be of consequence for ice formation in the whole Arctic

Water Mass Classification on a Highly Variable Arctic Shelf Region: Origin of Laptev Sea Water Masses and Implications for the Nutrient Budget

Large gradients and inter annual variations on the Laptev Sea shelf prevent the use of uniform property ranges for a classification of major water masses. The central Laptev Sea is dominated by predominantly marine waters, locally formed polynya waters and riverine summer surface waters. Marine waters enter the central Laptev Sea from the northwestern Laptev Sea shelf and originate from the Kara Sea or the Arctic Ocean halocline. Local polynya waters are formed in the Laptev Sea coastal polynyas. Riverine summer surface waters are formed from Lena river discharge and local melt. We use a principal component analysis (PCA) in order to assess the distribution and importance of water masses within the Laptev Sea. This mathematical method is applied to hydro-chemical summer datasets from the Laptev Sea from five years and allows to define water types based on objective and statistically significant criteria. We argue that the PCA derived water types are consistent with the Laptev Sea hydrography and indeed represent the major water masses on the central Laptev Sea shelf. Budgets estimated for the thus defined major Laptev Sea water masses indicate that freshwater inflow from the western Laptev Sea is about half or in the same order of magnitude as freshwater stored in locally formed polynya waters. Imported water dominates the nutrient budget in the central Laptev Sea; and only in years with enhanced local polynya activity is the nutrient budget of the locally formed water in the same order as imported nutrients