Tõnu Martma

Evidence for a widespread carbon isotopic event associated with late Middle Ordovician sedimentological and faunal changes in Estonia

An episode of remarkable biotic, climatic, sea-level and facies changes took place during the late Viruan (Caradoc) epoch in the Baltoscandian area. We studied the carbon isotopic composition of carbonate sediments from this period. Data on the stable carbon isotopic composition of whole-rock carbonates from three south Estonian core sections, together with those on ostracode, distribution are presented. In two core sections, a positive δ 13 C shift of 2 ‰ was revealed in the upper part of the Keila Stage (mid-Caradoc). The comparison of isotope and ostracode data in the sections suggested the occurrence of a gap of late Keilan age in the marginal area of the basin equivalent to the North Estonian Confacies Belt. The beginning of the excursion approximately coincided with climatic and sea-level changes in the shallow shelf area. The peak of the late Keilan excursion preceded the biotic crises and maximum black shale accumulation in the Baltoscandian palaeobasin. An approximately synchronous carbon isotopic event has been reported from North America, referring to a possible global oceanographic event in the Caradoc epoch. Although the positive carbon isotopic excursion and related environmental events of the late Keilan age have some unique features, they show more similarities to the end-Ordovician and Silurian events, characterized by oceanic change from stratified state to thermohaline circulation state, than to warm anoxic events related to eustatic sea-level rise.

Isotopic measurements of precipitation on central Asian glaciers (Southeastern Tibet, northern Himalayas, central Tien Shan)

The glacial regions of central Asia considered in our study are influenced by tropical monsoons and western extratropical cyclones. Isotopic δ18O and δD data were obtained over 3 years in three climatic regions: Gongga massif of southeastern Tibet (windward slope of summer monsoon), Xixibangma massif on the northern slope of the Himalayas (leeward of summer monsoon), and the massifs of Pobeda-Khan Tengry in the central Tien Shan (exposed to western airstreams). The survey reported here provides information from atmospheric precipitation, snow pits and a 23 m ice core. The significant differences of oxygen isotopic ratios (from −25.1 per mil to −9.5 per mil) indicate that the Indian and Pacific Oceans as well as the Atlantic Ocean are sources of moisture on the northern slope of the Himalayas and southeastern Tibet. Sharply changing isotopic ratios in precipitation corresponded to changing wind direction and were associated with different sources of air masses on the northern slope of the Himalayas and southeastern Tibet. Steady isotopic ratios in atmospheric precipitation and the absence of changing δ18O composition in a 23 m ice core suggest only one source of moisture in central Tien Shan. The relatively heavier oxygen isotopic ratios in atmospheric precipitation and ice core indicate the moisture from which the precipitation was derived originated over the Caspian or Mediterranean seas. Within the Eurasian continent, air masses developed over the Atlantic Ocean advance farther than those from the Pacific and Indian Oceans.

Effect of periodic melting on geochemical and isotopic signals in an ice core from Lomonosovfonna, Svalbard

[1] We examine the quality of atmospherically deposited ion and isotope signals in an ice core taken from a periodically melting ice field, Lomonosovfonna in central Spitsbergen, Svalbard. The aim is to determine the degree to which the signals are altered by periodic melting of the ice. We use three diagnostics: (1) the relation between peak values in the ice chemical and isotopic record and ice facies type, (2) the number of apparent annual cycles in these records compared with independently determined number of years represented in the ice core, and (3) a statistical comparison of the isotopic record in the ice core and the isotope records from coastal stations from the same region. We find that during warm summers, as much as 50% of the annual accumulation may melt and percolate into the firn; in a median year this decreases to similar to25%. As a consequence of percolation, the most mobile acids show up to 50% higher concentrations in bubble-poor ice facies compared with facies that are less affected by melt. Most of the other chemical species are less affected than the strong acids, and the stable water isotopes show little evidence of mobility. Annual or biannual cycles are detected in most parameters, and the water isotope record has a comparable statistical distribution to isotopic records from coastal stations. We conclude that ice cores from sites like Lomonosovfonna contain a useful environmental record, despite melt events and percolation and that most parameters preserve an annual, or in the worst cases, a biannual atmospheric signal.

Implications of Gondwana glaciations in the Baltic late Ordovician and Silurian and a carbon isotopic test of environmental cyclicity

Four glaciations -- the extensively studied Hirnantian episode and three in the Llandovery-earliest Wenlock -- are well established on the Gondwana palaeocontinent. New data [Hamoumi, 1999] shift the beginning of the glacial epoch to the early Caradoc when Baltica was moving from middle to low latitudes of the southern Hemisphere. Despite the rather considerable distance between polar areas of Gondwana and subtropical Baltica all four glacial events are reflected via global climatic mechanisms in the East Baltic. It is generally accepted that glaciations are marked by positive excursions of {delta}18O and {delta}13C values caused by increase of the polar ice caps, bioproduction and decrease of oceanic water temperature, etc. Based on these relationships, the Gondwana glacial events are correlated with coeval isotopic shifts established in sections of Baltica. In addition, agreement of the oceanic processes and corresponding carbon isotopic trends predicted by Jeppsson [1990] to real measured values is analysed. The following positive {delta}13C excursions are recorded in the Baltic area (peak values in parentheses) : middle Caradoc (2.2 {per thousand}), early Ashgill (2.5 {per thousand}), Hirnantian (6 {per thousand}), early Aeronian (3.7 {per thousand}), early Telychian (2.7 {per thousand}), early Wenlock (5.2 {per thousand}). Most of these shifts correlate well with glacio-eustatic sea level lowstands and biodiversity changes, as shown by the most extensive Oandu crisis in the Caradoc, Hirnantian mass extinction and the Wenlock Ireviken Event. Analysis of data allows the following conclusions : (1) all four Gondwana glaciations identified by tillites, microconglomeratic clays, etc. and dated biostratigraphically are recognised in the Baltic area through clear positive carbon isotope excursions at the same levels; (2) three smaller carbon isotope excursions in the Caradoc and Ashgill together with algal abundance data suggest the presence of several colder climate episodes during the late Ordovician. This may support the idea of the earlier onset of the glacial epoch on Gondwana; however, correct biostratigraphic dating of supposed glacial sediments is required; (3) the carbon isotopic testing of the oceanic model by Jeppsson reveals too many contradictions between model predictions and measured values. This means that the environmental background of isotopic events and relationships with oceanic events should be revised ; (4) for delimitation of the climatic --oceanic episodes, a more general marker identifying environmental change via a basinal approach seems useful. For this purpose lithological, geochemical or palaeontological criteria can be used.

A new ice-core record from Lomonosovfonna, Svalbard: viewing the 1920-97 data in relation to present climate and environmental conditions

A new ice core record from Lomonosovfonna, Svalbard: viewing the data between 1920-1997 in relation to present climate and environmental conditions

Reconstruction of three centuries of annual accumulation rates based on the record of stable isotopes of water from Lomonosovfonna, Svalbard

Abstract We use the upper 81 mof the record of stable isotopes of water from a 122m long ice core from Lomonosovfonna, central Spitsbergen, Svalbard, to construct an ice-core chronology and the annual accumulation rates over the icefield. the isotope cycles are counted in the ice-core record using a model that neglects short-wavelength and low-amplitude cycles. We find approximately the same number of δ18O cycles as years between known reference horizons, and assume these cycles represent annual cycles. Testing the validity of this assumption using cycles in δD shows that both records give similar numbers of cycles. Using the δ18O chronology, and decompressing the accumulation records using the Nye flow model, we calculate the annual accumulation for the ice-core site back to AD 1715. We find that the average accumulation rate from 1715 to 1950 was on average 0.30 mw.e. Accumulation rates increased about 25% during the later part of the 20th century to an average of 0.41 mw.e. for the period 1950–97. the accumulation rates show highly significant 2.1 and 21 year periodicities, which gives credibility to our time-scale.

Thousand years of winter surface air temperature variations in Svalbard and northern Norway reconstructed from ice-core data

Two isotopic ice core records from western Svalbard are calibrated to reconstruct more than 1000 years of past winter surface air temperature variations in Longyearbyen, Svalbard, and Vardø, northern Norway. Analysis of the derived reconstructions suggests that the climate evolution of the last millennium in these study areas comprises three major sub-periods. The cooling stage in Svalbard (ca. 800–1800) is characterized by a progressive winter cooling of approximately 0.9 °C century−1 (0.3 °C century−1 for Vardø) and a lack of distinct signs of abrupt climate transitions. This makes it difficult to associate the onset of the Little Ice Age in Svalbard with any particular time period. During the 1800s, which according to our results was the coldest century in Svalbard, the winter cooling associated with the Little Ice Age was on the order of 4 °C (1.3 °C for Vardø) compared to the 1900s. The rapid warming that commenced at the beginning of the 20th century was accompanied by a parallel decline in sea-ice extent in the study area. However, both the reconstructed winter temperatures as well as indirect indicators of summer temperatures suggest the Medieval period before the 1200s was at least as warm as at the end of the 1990s in Svalbard.

Timescales of methane seepage on the Norwegian margin following collapse of the Scandinavian Ice Sheet

Gas hydrates stored on continental shelves are susceptible to dissociation triggered by environmental changes. Knowledge of the timescales of gas hydrate dissociation and subsequent methane release are critical in understanding the impact of marine gas hydrates on the ocean–atmosphere system. Here we report a methane efflux chronology from five sites, at depths of 220–400 m, in the southwest Barents and Norwegian seas where grounded ice sheets led to thickening of the gas hydrate stability zone during the last glaciation. The onset of methane release was coincident with deglaciation-induced pressure release and thinning of the hydrate stability zone. Methane efflux continued for 7–10 kyr, tracking hydrate stability changes controlled by relative sea-level rise, bottom water warming and fluid pathway evolution in response to changing stress fields. The protracted nature of seafloor methane emissions probably attenuated the impact of hydrate dissociation on the climate system.

Svalbard summer melting, continentality, and sea ice extent from the Lomonosovfonna ice core

We develop a continentality proxy (1600–1930) based on amplitudes of the annual signal in oxygen isotopes in an ice core. We show via modeling that by using 5 and 15 year average amplitudes the effects of diffusion and varying layer thickness can be minimized, such that amplitudes then reflect real seasonal changes in δ18O under the influence of melt. A model of chemical fractionation in ice based on differing elution rates for pairs of ions is developed as a proxy for summer melt (1130–1990). The best pairs are sodium with magnesium and potassium with chloride. The continentality and melt proxies are validated against twentieth-century instrumental records and longer historical climate proxies. In addition to summer temperature, the melt proxy also appears to reflect sea ice extent, likely as a result of sodium chloride fractionation in the oceanic sea ice margin source area that is dependent on winter temperatures. We show that the climate history they depict is consistent with what we see from isotopic paleothermometry. Continentality was greatest during the Little Ice Age but decreased around 1870, 20–30 years before the rise in temperatures indicated by the δ18O profile. The degree of summer melt was significantly larger during the period 1130–1300 than in the 1990s.

The Icelandic Laki volcanic tephra layer in the Lomonosovfonna ice core, Svalbard

The largest sulphuric acid event revealed in an ice core from the Lomonosovfonna ice cap, Svalbard, is associated with the densest concentration of microparticles in the ice core at 66.99 m depth. Electron microscope analysis of a volcanic ash particle shows it has the same chemical composition as reported for debris from the eruption of Iceland’s Laki fissure in 1783 and confirms the identification of the tephra. Most of the particles in the deposit are not ash, but are common sand particles carried aloft during the eruption event and deposited relatively nearby and downwind of the long-lasting eruption. The tephra layer was found 10 - 20 cm deeper than high sulphate concentrations, so it can be inferred that tephra arrived to Lomonosovfonna about 6 - 12 months earlier than gaseous sulphuric acid precipitation. The sulphuric acid spike has a significant cooling impact recorded in the oxygen isotope profile from the core, which corresponds to a sudden drop in temperature of about 2 °C which took several years to recover to previous levels. These data are the first particle analyses of Laki tephra from Svalbard and confirm the identification of the large acidic signal seen in other ice cores from the region. They also confirm the very large impact that this Icelandic eruption, specifically the sulphuric acid rather than ash, had on regional temperatures.