ALBERT

Description: 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.

Description: The shells of the planktonic foraminifer Neogloboquadrina pachyderma have become a classical tool for reconstructing glacial–interglacial climate conditions in the North Atlantic Ocean1, 2, 3. Palaeoceanographers utilize its left- and right-coiling variants, which exhibit a distinctive reciprocal temperature and water mass related shift in faunal abundance both at present and in late Quaternary sediments1, 2, 4, 5. Recently discovered cryptic genetic diversity in planktonic foraminifers6, 7, 8 now poses significant questions for these studies. Here we report genetic evidence demonstrating that the apparent ‘single species’ shell-based records of right-coiling N. pachyderma used in palaeoceanographic reconstructions contain an alternation in species as environmental factors change. This is reflected in a species-dependent incremental shift in right-coiling N. pachyderma shell calcite δ18O between the Last Glacial Maximum and full Holocene conditions. Guided by the percentage dextral coiling ratio, our findings enhance the use of δ18O records of right-coiling N. pachyderma for future study. They also highlight the need to genetically investigate other important morphospecies to refine their accuracy and reliability as palaeoceanographic proxies.

Description: The upper 500 or 1000 m of the water column in the Okhotsk Sea was sampled for living planktic foraminifera. The polar species Neogloboquadrina pachyderma (sinistral) strongly dominates the foraminiferal assemblage; the subpolar to temperate species Globigerina bulloides accounts for 10–25%. Other species account for up to 3% only. The shell δ18O and δ13C values of the species N. pachyderma (sin.) are compared to water δ18O values and δ13C values of dissolved inorganic carbon (DIC). The strong gradient in δ18O composition and temperature in the upper water column is reflected in the δ18O of N. pachyderma (sin.). Relative to the values expected for inorganic calcite precipitated under equilibrium conditions N. pachyderma (sin.) displays a vital effect of about 1‰ in δ18O. The δ13C composition of N. pachyderma (sin.) is about constant with water depth and the reflection of δ13CDIC in the foraminiferal shell seems to be masked by other effects. Most foraminifera are found above or slightly below the thermocline and can be assumed to calcify in the upper 200 m of the water column. The gradient of δ13CDIC extends well below this depth, therefore the lack of correlation can partly be attributed to this fact. The remaining discrepancy between δ13C of N. pachyderma (sin.) and δ13CDIC correlates with the carbonate ion concentration in the water column. This leads to the conclusion that the ‘carbonate ion effect’ (CIE), which has been derived from culturing experiments for other species [Spero et al. (1997) Nature 390, 497–500], is found here under natural conditions. When the magnitude of the CIE derived for G. bulloides is applied to N. pachyderma (sin.), CIE-corrected δ13C of N. pachyderma (sin.) is a direct reflection of δ13CDIC in the water column with a constant offset of 1.2‰.

Description: Ostracods secrete their valve calcite within a few hours or days, therefore, its isotopic composition records ambient environmental conditions of only a short time span. Hydrographic changes between the calcification of individuals lead to a corresponding range (max.–min.) in the isotope values when measuring several (≥5) single valves from a specific sediment sample. Analyses of living (stained) ostracods from the Kara Sea sediment surface revealed high ranges of 〉2‰ of δ18O and δ13C at low absolute levels (δ18O: 〈3‰, δ13C: 〈−3‰) near the river estuaries of Ob and Yenisei and low ranges of ∼1‰ at higher absolute levels (δ18O: 2–5.4‰, δ13C: −3‰ to −1.5‰) on the shelf and in submarine paleo-river channels. Comparison with a hydrographic data base and isotope measurements of bottom water samples shows that the average and the span of the ostracod-based isotope ranges closely mirror the long-term means and variabilities (standard deviation) of bottom water temperature and salinity. The bottom hydrography in the southern part of the Kara Sea shows strong response to the river discharge and its extreme seasonal and interannual variability. Less variable hydrographic conditions are indicative for deeper shelf areas to the north, but also for areas near the river estuaries along submarine paleo-river channels, which act as corridors for southward flowing cold and saline bottom water. Isotope analyses on up to five single ostracod valves per sample in the lower section (8–7 cal. ka BP) of a sediment core north of Yenisei estuary revealed δ18O and δ13C values which on average are lower by 0.6‰ in both, δ18O and δ13C, than in the upper core section (〈5 cal. ka BP). The isotope shifts illustrate the decreasing influence of isotopically light river water at the bottom as a result of the southward retreat of the Yenisei river mouth from the coring site due to global sea level rise. However, the ranges (max.–min.) in the single-valve δ18O and δ13C data of the individual core samples are similar in the upper and in the lower core section, although a higher hydrographic variability is expected prior to 7 cal. ka BP due to river proximity. This lack of variability indicates the southward flow of cold, saline water along a submarine paleo-river channel, formerly existing at the core location. Despite shallowing of the site due to sediment filling of the channel and isostatic uplift of the area, the hydrographic variability at the core location remained low during the Late Holocene, because the shallowing proceeded synchronously with the retreat of the river mouth due to the global sea level rise.

Description: Modern processes are evaluated to understand the possible mechanisms behind last glacial benthic foraminiferal δ18O anomalies that occurred concurrent with meltwater events in the polar North Atlantic; such anomalies in the Nordic seas were recently interpreted to be caused by brine formation. Despite intensive sea-ice production on circumarctic shelves, modern data show that brines ejected from sea-ice formation containing low δ18O water do not significantly contribute to deep waters in the Arctic Ocean today. Assuming that this process was, nevertheless, responsible for δ18O anomalies in Nordic seas deep water during the last glaciation, a broad, shallow shelf area adjacent to the Nordic seas, such as the Barents Sea, had to be seasonally free of sea-ice in order to serve as an area for brine formation. Another process which may explain δ18O-depleted water at depth is found in the Weddell Sea today, where a low δ18O signal in deep waters originates from ice shelf interactions. If temperature were considered the main mechanism for the low benthic δ18O values, an increase of 4°C must have occurred in the deep water. An analogous situation with a reversed water temperature pattern due to a subsurface inflow of warm Atlantic water is found today in the eastern Arctic Ocean, and deep water warming is observed in the Greenland Gyre in the absence of deep convection. Because paleoproxy data also indicate an Atlantic water inflow into the Nordic seas during such benthic δ18O anomalies, temperature as a principal mechanism of changing δ18O cannot be excluded.

Description: 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.