Geochemical constraints on the origin of enigmatic cemented chalks, Norfolk, UK

Very hard cemented chalk stacks and crusts found locally in the upper part of the Cretaceous Chalk of north Norfolk, UK, are related to solution features. The solution features, mainly pipes and caves, formed after deposition of the overlying Middle Pleistocene Wroxham Crag, probably by routing of sub-glacial, or glacial, melt-waters derived from late Pleistocene glaciers. New geochemical (particularly stable isotope) data shows that cementation of the chalks, although related spatially to the solution features, was not caused by glacier-derived waters. The carbon isotope composition of the chalk cements is typically around -9.5‰, indicative of biologically active soils. Moreover, the oxygen isotope compositions of the cements, around -5‰, are incompatible with water d18O values much below -9 to -10‰ (which probably precludes isotopically negative glacier-derived water), as resulting palaeo-temperatures are below zero. Taken together, the isotope data suggest chalk cementation occurred under interglacial conditions similar to the present. Dissolved calcium carbonate for cementation came from dissolution of reworked chalk in overlying MIS 12 glacial tills

Isotope hydrology of dripwaters in a Scottish cave and implications for stalagmite palaeoclimate research

Dripwater hydrology and hydrogeochemistry is particularly useful in constraining the meaning of speleothem palaeoclimate archives, for example using δ<sup>18</sup>O signatures. Here, we calibrate the relationship between δ<sup>18</sup>O in precipitation, percolation waters and contemporary calcite deposits, at Tartair cave, Sutherland, NW Scotland, an Atlantic site sensitive to regional changes both of temperature and precipitation. Monthly precipitation displayed a 7.1‰ range in δ<sup>18</sup>O, a negative linear relationship with rainfall amount, and no correlation with temperature. Autogenically-derived cave percolation waters show little variation in δ<sup>18</sup>O during the same period and their annual weighted mean is the same as that of the local precipitation. This evidence together with hydrological data and electroconductivity values indicates that percolation waters are well mixed and dominated by stored water. Calculated values of δ<sup>18</sup>O of calcite deposited in this cave environment indicate that the cave deposits are forming close to isotopic equilibrium and kinetic effects are negligible. Comparison of a high-resolution δ<sup>18</sup>O stalagmite record with the instrumental record of climate indicates that isotopically heavy values are reflective of relatively cold, dry conditions (and vice-versa for warm, wet condition) and hence that stalagmite oxygen isotopes provide an appropriate means of investigating the palaeoclimate in this location

High resolution δ18O seasonality record in a French Eemian tufa stromatolite (Caours, Somme Basin)

International audienc

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipelike, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ 18O around +4‰ and δ 13C values between -8 and -29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period

A922 Sequential measurement of 1 hour creatinine clearance (1-CRCL) in critically ill patients at risk of acute kidney injury (AKI)

Meeting abstrac

Depositional continuity of seasonally laminated tufas: Implications for δ¹⁸O based palaeotemperatures

A ~100 ka old laminated tufa (freshwater carbonate) deposit from central Greece was studied for evidence of a seasonal origin of the laminae. Annual laminar couplets (6 mm thick) consist of dense β bands of spring– early summer calcified cyanobacterial bushes and porous α bands of autumn precipitated calcite on unclustered cyanobacterial filaments. This is evidence of likely seasonal palaeoclimate during the last interglacial in Greece. δ<sup>18</sup>O variability is explained by seasonal changes in stream water temperature; however, the total range in δ<sup>18</sup>O translates to ~6 °C (±1 °C) almost certainly underestimating the likely actual range of ~17 °C. As abrupt changes in δ<sup>18</sup>O inferred temperatures (from warming to cooling and vice versa) occur exactly on sharp petrographic lamina boundaries, we infer that mid-summer and mid-winter precipitates are missing. We suggest that when stream conditions (summer drying and winter cold) are unsuited to cyanobacterial substrate development, no tufa build-up occurs. Laminated tufas are thus incomplete records of annual tufa formation, as are the δ18O based palaeoclimate records they preserve. They require careful petrographic study linked to high resolution sampling for δ<sup>18</sup>O to ensure that palaeotemperature ranges are not seriously underestimated

Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains

In Zakynthos Island (Greece), authigenic cementation of marine sediment has formed pipe-like, disc and doughnut-shaped concretions. The concretions are mostly composed of authigenic ferroan dolomite accompanied by pyrite. Samples with >80% dolomite, have stable isotope compositions in two groups. The more indurated concretions have δ18O around +4‰ and δ13C values between -8 and -29‰ indicating dolomite forming from anaerobic oxidation of thermogenic methane (hydrocarbon seep), in the sulphate-methane transition zone. The outer surfaces of some concretions, and the less-cemented concretions, typically have slightly heavier isotopic compositions and may indicate that concretion growth progressed from the outer margin in the ambient microbially-modified marine pore fluids, inward toward the central conduit where the isotopic compositions were more heavily influenced by the seep fluid. Sr isotope data suggest the concretions are fossil features, possibly of Pliocene age and represent an exhumed hydrocarbon seep plumbing system. Exposure on the modern seabed in the shallow subtidal zone has caused confusion, as concretion morphology resembles archaeological stonework of the Hellenic period. © 2016 Elsevier Ltd