Carsten Israelson

Rapid hydrological changes during the Holocene revealed by stable isotope records of lacustrine carbonates from Lake Igelsjon, southern Sweden

A Holocene sediment sequence from Lake Igelsjon, south central Sweden, was studied by stable oxygen- and carbon-isotope analyses of different carbonate components. The deposit, which covers the time-span from ca 11,500 cal BP to the present, was laid down in a small, kettle-hole lake, the hydrological balance of which is presently dominated by groundwater flow. Isotopic records obtained on bulk carbonates originating mainly from summer-produced, calcitic algal encrustations exhibit several rapid shifts of more than 2parts per thousand, likely reflecting pronounced hydrological variations. Corresponding isotopic data obtained on calcitic gastropod opercula from parts of the profile show subdued responses to major climatic shifts, probably related to an extended calcification season. The isotopic records were complemented by studies of modern isotope hydrology, and our interpretations are based on a simplistic climate-hydrology model in which variations in groundwater generation within the lake catchment produce changes in groundwater level and related adjustments of lake level and surface/volume ratio of the basin during the ice-free season. Assumed periods of decreased lake volume in a relatively dry climate (low lake level) are characterised by enrichment in O-18 and C-13 resulting from increased evaporation/inflow ratio and atmospheric equilibration, respectively. In clear contrast to this situation, intervals of more humid climatic conditions give rise to increased lake volume (high lake level), possibly surface over flow, and relatively depleted isotopic ratios. Relatively humid conditions, which may correlate to a wide-spread cooling event recorded by various proxies across the North Atlantic region, are indicated by distinct isotopic shifts at ca 8300 and 8000 cal BP, bracketing a period of O-18-depletion. The period between ca 8000 and 4000 cal BP was characterised by relatively dry and stable climatic conditions, whereas the subsequent part of the Holocene experienced a more humid and variable climate following marked and coherent depletions in O-18 and C-13 at ca 4000 cal BP

Ikaite Tufa Towers in Ikka Fjord, Southwest Greenland: Their Formation by Mixing of Seawater and Alkaline Spring Water

ABSTRACT Ikka Fjord in southwest Greenland is the type locality for ikaite, a metastable hexahydrate of calcium carbonate. Here, ikaite forms submarine tufa columns rising up to 18 m above the fjord bottom. The columns are spectacular examples of abiogenic carbonate precipitation in a cold seawater environment and so far represent the only known formation of ikaite in seawater. We have analyzed the 2H, 13C, 14C, 18O and 87Sr contents and major-element and minor-element compositions of ikaite and of the different water types in and around Ikka Fjord in order to understand the formation of the Ikka columns. Water from inside the columns is fresh but alkaline, rich in ions of sodium, bicarbonate, and carbonate, and has a stable-isotope composition identical to precipitation falling on the nearby Gronnedal-Ika igneous complex, and we conclude that the tufa columns form over submarine springs issuing from the bottom of Ikka Fjord. Dissolution of unidentified sodium carbonate minerals in carbonatites in the igneous complex probably accounts for the peculiar chemistry of the springs. Ikaite supersaturation and precipitation is controlled by mixing of spring water and seawater at the cold (< 6°C) fjord bottom, and precipitation is accelerated by the formation of hydrated CaCO3o (aq) ion pairs, while phosphate ions in the spring water act as inhibitor for precipitation of anhydrous carbonates. We propose that the spring water seeps in Ikka Fjord literally create their own conduits in the form of the vertical, chimney-like columns with ikaite forming continuously at the tops of the columns. The only limits to upward growth are winter ice and the fresh-water layer capping the fjord during summer.

A multi-proxy study of Pliocene sediments from Île de France, North-East Greenland

Abstract A multi-technique approach has been used to study a Pliocene shallow water marine deposit, designated the Ile de France Formation, in North-East Greenland. The sequence is correlated on the basis of 87Sr–86Sr ratios in shells and palaeomagnetic studies with the Gauss normal polarity chron, which is dated to between 2.60 and 3.58 Ma years BP. This dating is in accordance with amino acid epimerisation and evidence from dinoflagellates, foraminifers and molluscs. Sediments, marine molluscs and foraminifers show that the sequence was deposited on the inner shelf, below storm wave base. Seawater temperatures were much higher than today, as demonstrated by the occurrence of a number of southern extra-limital species. The same applies to air temperature, and the few remains of land plants may indicate a forested upland with Picea and Thuja. A number of extinct taxa are present, including Nucula jensenii that is erected as a new species.

Oxygen and carbon isotope composition of Quaternary bivalve shells as a water mass indicator: Last interglacial and Holocene, East Greenland

Abstract Oxygen and carbon isotope composition of arctic bivalve shells are used in an attempt to reconstruct surface water temperature and salinities in Scoresby Sund, East Greenland. The oxygen isotope compositions of Mya truncata , Hiatella arctica and Tridonta borealis have been compared with present day hydrological parameters. Modern shells yield oxygen isotope values that, on the whole, reproduce the environmental temperature and sea water isotopic composition. Furthermore, it is possible to estimate the living depth of the analysed specimens. Analyses of growth increments from single shells show that there are large variations from year to year in temperature and oxygen isotope composition of the surface waters of Scoresby Sund and that these variations decrease with depth. Analyses of Holocene shells indicate that the Polar Current water, which flows from north to south along the East Greenland coast was also present during the Holocene climatic optimum 8000-7000 yr B.P. Analyses of bivalve shells from the last interglacial show that Scoresby Sund during that time was well circulated and that meltwater from the Greenland ice sheet and sea ice meltwater was important for the temperature, salinity and isotopic composition of the surface waters.

Strontium and oxygen isotopic composition of East Greenland rivers and surface waters: implication for palaeoenvironmental interpretation

Abstract Isotopic composition of strontium and oxygen and strontium concentrations from 4 hydrographic sites in Scoresby Sund Fjord and 6 rivers draining the adjacent Jameson Land have been investigated. Schuchert Flod, the major river on Jameson Land, erodes a large celestite (SrSO 4 ) deposit. Dissolved Sr in Schuchert Flod has 87 Sr/ 86 Sr = 0.7137 and is responsible for most of the continental Sr flux to the fjord. The Sr isotope data have been used to construct a mixing model for the surface water in Scoresby Sund Fjord. Salinity ( S ) and Sr concentrations ( C Sr ) of brackish water from the fjord show conservative mixing between river water and seawater. Results of the 87 Sr/ 86 Sr–salinity mixing model of seawater and fresh water were used to interpret the isotopic composition of Sr ( 87 Sr/ 86 Sr) and oxygen (δ 18 O c ) of recent and interglacial (Eemian ≈120 ka) shallow-water bivalve shells from the coast of Jameson Land. Because of the high C Sr in some of the rivers draining Jameson Land, a correlation exists between 87 Sr/ 86 Sr and δ 18 O c of recent and interglacial shallow water bivalve shells from the coast of Jameson Land. Higher-than-seawater 87 Sr/ 86 Sr values in shells from the Langelandselv interglaciation can be explained only if the shells were formed close to a Sr-rich fresh water source which is not present in the Langelandselv area today. These results suggest that there was a different river and drainage system on the Jameson Land peninsula during the Last Interglacial. The 87 Sr/ 86 Sr values indicate that shell formation took place in waters with salinities between 20 and 31‰ in a fjord with less glacial melt-water than seen today.