Benthic ostracode ∂13C as sensor for early Holocene establishment of modern circulation patterns in Central Europe
2013 (English)In: Quaternary Science Reviews, ISSN 0277-3791, Vol. 66, no SI, 112-122 p.Article in journal (Refereed) Published
Shells from adult specimen of the benthic ostracodes Limnocytherina sanctipatricii and Leucocythere mirabilis selected from a 8.7¬†m long piston core provide continuous stable oxygen and carbon records for the past approximately 16¬†ka. Oxygen isotopes from both species show identical values and track the general North Atlantic and European temperature history since deglaciation in great detail. Values of ostracode Œ¥18O values suggest that about 16¬†cal¬†ka the average annual air temperatures were about 11¬†¬∞C colder than today. Carbon isotopic values from both species of ostracodes are similar during the Lateglacial and early Holocene, and show an overall decrease from¬†‚àí4‚Ä∞ to¬†‚àí7‚Ä∞ that is probably related to an increase in photosynthetic productivity in the water column, as suggested by an increase in organic carbon, delivering 13C-depleted organic matter to the bottom waters (carbon pump). About 9¬†cal¬†ka only L.¬†mirabilis Œ¥13C values decreased about¬†‚àí2.5‚Ä∞ within 300 years. Higher Œ¥13C variability and ecological evidence suggests that L.¬†mirabilis represents a summer signal, whereas L.¬†sanctipatricii displays a more subdued annual average. After about 7¬†cal¬†ka another¬†‚àí1.5% decrease for both species, accompanied by an increase in magnetic susceptibility, a decrease in carbonate content, and more positive bulk carbonate isotope values followed, suggesting higher detrital-clastic input into the lake. In order to provide a possible mechanism explaining the negative L.¬†mirabilis Œ¥13C-values, sediment pore water profiles of O2 and CH4 in short cores collected from sites distal to proximal to the Alpine Rhine River delta, were inspected. Sediments in cores from more proximal sites to the Rhine delta become anoxic at shallower sediment depth due to the decay of high allochthonous organic carbon input to the sediment, which greatly increases concentrations of methane in pore waters closer to the Rhine inflow. When methane is oxidized close to the sediment‚Äìwater-interface, 13C-depleted carbon is added to pore water DIC that is then available for incorporation into ostracode shells. This mechanism suggests that about 9¬†cal¬†ka the oxygen supply to the bottom waters, especially in summer, decreased. This stimulated methanogenesis close to the sediment‚Äìwater-interface, and provided Œ¥13C-depleted carbon to benthic dwellers. Independent evidence for methanogenesis is provided by the increase in concentration of tetrahymanol after about 9¬†cal¬†ka coincident with the decrease in Œ¥13C of L.¬†mirabilis. We suggest that about 9¬†cal¬†ka the northward retreat of the Northern Hemisphere Ice Sheets, and consequently the polar front, left the alpine region affected by a more oceanic climate, characterized by warmer winters as they occur today especially during the positive North Atlantic Oscillation Index phase. More frequently incomplete mixing of the water column may have shifted the decay of organic matter faster to anaerobic conditions in surficial sediments especially during summer. By about 7¬†cal¬†ka the North Atlantic region had probably warmed sufficiently to increase precipitation in Central Europe and consequently detrital-clastic runoff to Lake Constance.
Place, publisher, year, edition, pages
2013. Vol. 66, no SI, 112-122 p.
Stable carbon and oxygen isotopes, Lake sediments, Ostracodes, Methanogenesis, Tetrahymanol, North Atlantic Oscillation, Central Europe, Early Holocene
IdentifiersURN: urn:nbn:se:uu:diva-191101DOI: 10.1016/j.quascirev.2012.10.032ISI: 000317448500011OAI: oai:DiVA.org:uu-191101DiVA: diva2:584776