[Show abstract][Hide abstract] ABSTRACT: Abrupt cooling events are features of Holocene climate and may recur in the future. We use lake records from Hawes Water, NW England, to quantify the impact of two prominent early Holocene climatic events. Subdecadal oxygen isotope records from sedimentary carbonate (18deltaOc), dated using thermal ionization mass spectrometry (TIMS) U-series analyses, provide evidence for abrupt cold events, lasting ˜50 and ˜150 yr at 9350 and 8380 yr ago, which correlate with the 9.3 ka and 8.2 ka events recognized in Greenland ice cores. At Hawes Water, mean July air temperatures, inferred from chirono-mid assemblages, decreased by ˜1.6 °C during each event. Calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation (18deltaOp) by ˜1.30/00; this is interpreted as a direct downstream response to cooling and freshening of northeast Atlantic surface water by melting ice sheets. Intermediate in magnitude between events observed in Greenland and central Europe, the effects are consistent with a partial shutdown of the North Atlantic thermohaline circulation.
[Show abstract][Hide abstract] ABSTRACT: Scanning electron microscopy of sediments, microfossils, and polished thin sections from the Cenomanian-Turonian boundary section at Dover, England, demonstrates that cement is concentrated within large pores such as foraminifera and calcispheres, whereas the surrounding coccolith-rich sediments are largely lacking in cement. Isotope analysis of single microfossils and cement samples indicates the presence of mixing lines between primary (paleoceanographic) compositions and cements. Consequently, paleoceanographic and stratigraphic interpretations of these sediments, and perhaps other similar sediments based on similar data, may be suspect.
[Show abstract][Hide abstract] ABSTRACT: A modified method for the carbon and oxygen isotope analysis of "small samples' (<100 μg) of common, naturally occurring carbonate minerals (calcite, dolomite, siderite) using conventional phosphoric acid digestion techniques is described. Samples are reacted in vacuo with ca. 30 μL of anhydrous acid and the resultant sample CO2 is processed for analysis via a purpose-built, manual gas clean-up line which is attached directly to the inlet of a stable isotope ratio mass spectrometer. Comparisons between isotopic values obtained from well characterized calcite, dolomite and siderite using both conventional (requiring 2–3 mg) and "small sample' analytical techniques show that reliable isotopic data can be obtained from samples weighing as little as 30 μg. At sample weights of <30 μg instrumental problems are encountered which result in significantly reduced levels of accuracy and precision. Although this technique is less efficient in terms of analysis time than commercial auto-preparation carbonate devices (also capable of processing ca. 30 μg samples), the procedures described here have a distinct advantage in that they allow analysis of a range of carbonate minerals which react slowly (hours to days) at "normal' (50–90 °C) reaction temperatures without a significant deterioration in precision or accuracy. This ability to analyse a range of carbonate minerals allows the potential benefits of high resolution sampling to be extended to a wider field of geochemical applications.
Preview · Article · Jun 1996 · Rapid Communications in Mass Spectrometry