Bjørn Buchardt

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

Holocene changes in atmospheric circulation recorded in the oxygen-isotope stratigraphy of lacustrine carbonates from northern Sweden

The oxygen-isotope composition of local precipitation (d18OP) is reconstructed from carbonate lake-sediment components in a sediment core covering the last 10000 calendar years from Lake Tibetanus, a small, hydrologically open, groundwater-fed lake in the Abisko area, northern Sweden. Comparison of the d18OP history with a pollen-based palaeotemperature record from the same core clearly reveals pronounced deviations from the normally expected temporal d18OP-temperature relation (so-called ‘Dansgaard relation’) that may be a function of changing oceanicity. The transition from relatively moist, maritime conditions in the early Holo cene to a much drier climate after 6500 cal. BP is re‘ ected by major changes in forest extent and composition as recorded by pollen and plant macrofossil data. At the time of maximum in‘ uence of westerly air-mass circulation (high zonal index) c. 9500 cal. BP, brought about by high summer insolation and enhanced meridi onal pressure gradients, d18OP at Lake Tibetanus was about 2‰ higher than would be predicted by the modern isotope-temperature relation. The occurrence of long-term changes in d18OP-temperature relations, which are more sensitive measures of palaeoclimate than either d18OP or temperature alone, needs to be taken into account when extracting palaeoclimatic information from continental oxygen-isotope records.

Submarine columns of ikaite tufa

In the small Ikka Fjord in southwestern Greenland, we have studied remarkable submarine tufa columns forming over alkaline springs by abiotic precipitationof the metastable, cold-water mineral ikaite (CaCO3.6H2O). The columns, which support a rich marine life, are up to 20 m high, with annual growth of more than 50 cm. We wish to nominate the locality, which seems to be unique, for classification as a geological World Heritage Site.

Multi-component carbon isotope evidence of early Holocene environmental change and carbon-flow pathways from a hard-water lake in northern Sweden

A 9000-year carbonate-rich sediment sequence from a small hard-water lake in northernmost Sweden was studied by means of multi-component stable carbon isotope analysis. Radiocarbon dating of different sediment fractions provides chronologic control and reveals a rather constant hard-water effect through time, suggesting that the lake has remained hydrologically open throughout the Holocene. Successive depletion of 13C in fine-grained calcite and carbonate shells during the early Holocene correlate with a change in catchment vegetation from pioneer herb communities to boreal forest. The vegetational change and associated soil development likely gave rise to an increased supply of 13C-depleted carbon dioxide in groundwater recharging the lake. This process is therefore believed to be the main cause of decreasing values of δ13C in dissolved inorganic carbon of the lake and thereby in limnic carbonates. Strongly 13C-depleted sedimentary organic matter may be related to enhanced kinetic fractionation during photosynthetic assimilation by means of proton pumping in Characean algae. This interpretation is supported by a substantial offset between δ13C of DIC as recorded by mollusc shells and δ13C of fine-grained calcite.

Strontium Uptake in Shell Aragonite from the Freshwater Gastropod Limnaea stagnalis

Shell aragonite from 96 specimens of the freshwater gastropod Limnaea stagnalis grown in laboratory tanks at different temperatures in water with variable strontium/calcium ratios have been analyzed for its strontium content in order to evaluate the mechanisms of strontium uptake in molluskan aragonite. Within the limits defined by natural freshwater environments, the strontium/calcium ratio in the aragonite was found to be linearly related to the strontium/calcium ratio in the water. A distribution coefficient kASr = 0.237 � 0.029, unaffected by variations in temperature and growth rate, has been found. This finding substantiates the existence of a strontium-discriminating effect in aragonite precipitated by mollusks as compared to the case for nonbiogenic aragonite which contains about five times as much strontium when precipitated under the same conditions.

Climate and environment during the Younger Dryas (GS-1) as reflected by composite stable isotope records of lacustrine carbonates at Torreberga, southern Sweden

Climatic and environmental changes during the Younger Dryas stadial (GS-1) and preceding and following transitions are inferred from stable carbon and oxygen isotope records obtained from the sediments of ancient Lake Torreberga, southern Sweden. Event GS-1 is represented in the sediment sequence by 3.5 m of clay containing lacustrine carbonates of various origins. Comparison of isotopic records obtained on mollusc shells, ostracod valves, and Chara encrustations precipitated during specific seasons of the year supports estimates of relative changes in both lake water and mean annual air temperatures. Variations in soil erosion rates can also be estimated from a simple isotope-mass-balance model to separate allochthonous and autochthonous carbonate contributions to the bulk carbonate content of the sediments. The well-known, rapid climatic shifts characterising the Last Termination in the North Atlantic region are clearly reflected in the isotopic data, as well as longer-term changes within GS-1. Following maximum cooling shortly after the Allerod-Younger Dryas (GI-1-GS-1) transition, a progressive warming and a slight increase in aquatic productivity is indicated. At the Younger Dryas- Preboreal (GS-1-PB) transition mean annual air temperature rapidly increased by more than 5°C and summer lake-water temperature increased by ca. 12°C. The subsequent Preboreal oscillation is characterised by an increase in soil erosion and a slight decrease in mean annual air temperature. These results are in harmony with recent findings about large-scale climate dynamics during the Last Termination. Copyright

Carbon isotope composition of lower palaeozoic kerogen: Effects of maturation

Abstract The carbon isotope composition of type I kerogen from a marine, organic-rich black shale deposit of Late Cambrian age (the Alum Shale Formation of southern Scandinavia) varies from −28 to −30% PDB. The thermal maturity is rated by HI-OI index, T max , PI and vitrinite reflectance values and published conodont alteration data. The results range from immature conditions with regard to oil generation in central Sweden to overmature and submetamorphic conditions in southern Sweden, Denmark and Norway. In the immature areas, the shale is found to be extremely hydrogen rich and oil prone (hydrocarbon potential up to 85 kg per ton rock). The observed δ 13 -variations are not correlated with maturity. It is therefore concluded that thermal maturation processes have no influence on the isotopic composition of a purely marine kerogen. The observed range in Lower Palaeozoic δ 13 (kerogen)-values (−27 to −31%) is in accordance with published results from other sedimentary basins.

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.

Oxygen isotope disequilibrium precipitation of calcite in Lake Arresø, Denmark

Lake carbonates are generally assumed to precipitate at or very close to oxygen isotopic equilibrium with the ambient water. We present results from the eutrophic Lake Arreso, northeastern Sealand, Denmark, that demonstrate disequilibrium precipitation of present-day calcite. According to the temperature equation of Epstein et al., these calcites are depleted in 18 O by 2‰–3‰ when compared to the expected equilibrium values. Core studies indicate that the development of disequilibrium conditions in Lake Arreso is correlated with the onset of an enhanced eutrophication. A high precipitation rate is suggested as the mechanism responsible for the observed 18 O depletion of the calcite precipitates.

UPb dating of calcite concretions from Cambrian black shales and the Phanerozoic time scale

The UPb dating of carbonate offers considerable potential for accurate and direct dating of sedimentary rocks. Here we show that carbonate concretions from the Upper Cambrian black shales in southern Scandinavia have remarkably high U to Pb ratios (238U204Pb up to 288,000) permitting precise geochronology. A fine-crystalline early diagenetic concretion yields an age of 509.8 ± 5.1 Ma, providing an accurate age for the late Cambrian, whereas a coarse ‘cone-in-cone’ concretion has yielded an age of 478.2 ± 4.9 Ma, defining the final stage of sediment compaction and indicating a protracted dewatering and diagenetic history. These results demonstrate the power of this method for directly calibrating the geological time scale and constraining diagenetic histories.