High-resolution Stable Isotope Record Research Articles

Chemo- and biostratigraphy of the Gaojiashan section (northern Yangtze platform, South China): a new Pc-C boundary section

Abstract. The widespread, terminal Ediacaran Dengying Formation (~ 551–~ 542 Ma) of South China hosts one of the most prominent negative carbonate carbon isotope excursions in Earth's history and thus bears on the correlation of the Precambrian–Cambrian boundary worldwide. The dominantly carbonate strata of the Dengying Formation are largely studied for their unique preservation of its terminal Ediacaran fauna but their geochemical context is poorly known. This study presents the first high-resolution stable isotope record (δ13C, δ18O) of calcareous siliciclastic shallow-water deposits of the Gaojiashan section (Shaanxi Province). The section includes (in ascending order) the Algal Dolomite Member, the Gaojiashan Member and the Beiwan Member of the Dengying Formation. Our data record a major δ13Ccarb negative excursion to −6 ‰ in the uppermost Gaojiashan Member which is comparable in shape and magnitude to the global Precambrian–Cambrian boundary negative δ13C excursion. Our data set is consistent with a "shallow-water anoxia" scenario which is thought to contribute to the "Cambrian explosion". The stratigraphic occurrence of Cloudina and a large negative δ13C excursion suggest that the Precambrian–Cambrian boundary is located near the top of the Gaojiashan Member and, consequently, that overlying carbonates and dolomites of the Beiwan Member are of earliest Cambrian age. Thus the Gaojiashan section may represent a new shallow-water section spanning the Precambrian–Cambrian boundary. Although bio- and chemostratigraphic data support this novel interpretation, we cannot exclude the possibility that the key excursions may represent a local perturbation indicating a restricted-basin environment.

The Wunstorf Drilling Project: Coring a Global Stratigraphic Reference Section of the Oceanic Anoxic Event 2

The mid-Cretaceous greenhouse world (Albian–Turonian) was characterized by high atmospheric carbon dioxide levels, much higher global temperatures than at present, and and a lack of permanent ice caps at the poles (Bice et al., 2006; Huber et al., 2002; Wilson et al., 2002). A characteristic feature of this greenhouse world was the deposition of black shales during the Cenomanian / Turonian boundary interval (CTBI; Schlanger and Jenkyns, 1976; Arthur et al., 1990). These carbon-rich sediments reflect a major perturbation of-rich sediments reflect a major perturbation ofrich sediments reflect a major perturbation of the global carbon cycle, with an associated global Ocean Anoxic Event (OAE 2). Despite twenty years of research, thetwenty years of research, the years of research, the OAE 2 still remains critical for the understanding of some of the mechanisms driving the climate and ocean systems during this period of extreme warmth. Numerous Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites and many on-shore sections throughout the world have recovered sediments of CTBI age. However, many of theseHowever, many of theseany of these CTBI records and OAE 2 suffer from relatively poor stratigraphic control, which greatly reduces the options to which greatly reduces the options towhich greatly reduces the options togreatly reduces the options toes the options to the options to correlate paleotemperatures and carbon isotope records to micro- and macropaleontological findings on a high-resolution scale. In order to fill this gap, an onshore drilling project, funded by the Deutsche Forschungsgemeinschaft (DFG), has been initiated. In March 2006, an 80-m-long-m-longm-long-longlong core, covering the CTBI, has been successfully recovered near Wunstorf (20 km east of Hannover, Germany, Fig. 1). The easily accessible Wunstorf section is a highly expanded, middle Cenomanian to Cenomanian toCenomanian to earlyTuroniansuccessionexposingathick TuroniansuccessionexposingathickTuronian succession exposing a thick black shale sequence. The Wunstorf drilling site was placed next to a former quarry which supplied abundant and well preserved macro- and microfossils. The Wunstorf drilling project aims at establishing a high resolution stable isotope record for the black shale succes-sion (OAE 2) of the CTBI and developinging this into a globally applicable high resolu-tion bio- and chemostratigraphic reference section. Disciplines involved include micropaleontology (calcareous nannofossils, planktonic foraminifera), macro-paleontology (ammonites, inoceramids), stable isotopes and cyclostratigraphy mainly based on borehole logging, multi sensor core logging, and x-ray fluorescence (XRF) scanning and x-ray fluorescence (XRF) scanning data. The combination of geochemical, paleontological, and and logging data will allow high resolution chemo- and biostra-tigraphy for the CTBI which may in the future serve as an which may in the future serve as anthe future serve as anfuture serve as an international standard.

Chronology and paleoenvironment of Marine Isotope Stage 3 from two high-elevation speleothems, Austrian Alps

A new high-resolution stable isotope record from the alpine Kleegruben Cave (2165 m, Central Alps) is presented. This record largely duplicates a previously reported speleothem record from this site. High-precision U-series thermal ionization mass spectrometry dates constrain the growth history of this new sample (56–48 ka) to the Marine Isotope Stage (MIS) 3. Both stalagmites reveal highly similar variations in O isotopes that can be directly compared to O isotope variability associated with Dansgaard-Oeschger interstadials recorded in the Greenland ice cores (Greenland Interstadials, GIS). Based on the new record we refine our previous age assignments of the GIS during this time interval, i.e. GIS 15b at 55.7 ka, GIS 15a at 55.3 ka, onset (mid-point) of the prominent GIS 14 at 54.5 yr and onset of GIS 12 at ∼48 ka. The overall uncertainties associated with these ages are 0.1–0.3 ka. The timing of these Dansgaard-Oeschger interstadials is within 0.2 ka of the newly proposed GRIP.SFCP04 timescale. The age data for both Kleegruben stalagmites demonstrate that liquid water was at least seasonally present in the shallow sub-surface allowing calcite precipitation to continue even during stadials of MIS 3. Given the present-day low temperature (+2.4 °C) we propose that these unusual speleothems formed in a karst system overlain by a warm-based glacier.

Early Maastrichtian stable isotopes: changing deep water sources in the North Atlantic?

We propose that the observed short-term stable isotope fluctuations reflect changes in high- and low-latitude intermediate to deep water sources, based on a high-resolution stable isotope record of planktic and benthic foraminifera from the Early Maastrichtian (∼71.3 to ∼69.6 Ma) of Blake Nose (DSDP Site 390A, North Atlantic). Sources of these waters may have been the low-latitude eastern Tethys and high-latitude North Atlantic. Changes in intermediate to deep water sources were probably steered by eccentricity-controlled insolation fluctuations. Lower insolation favored the formation of high-latitude deep waters due to positive feedback mechanisms resulting in high-latitude cooling. This led to a displacement of low-latitude deep waters at Blake Nose. Higher insolation reduced intermediate to deep-water formation in high latitudes, yielding a more northern flow of low-latitude deep waters.

Early Maastrichtian stable isotopes: changing deep water sources in the North Atlantic?

We propose that the observed short-term stable isotope fluctuations reflect changes in high- and low-latitude intermediate to deep water sources, based on a high-resolution stable isotope record of planktic and benthic foraminifera from the Early Maastrichtian (∼71.3 to ∼69.6 Ma) of Blake Nose (DSDP Site 390A, North Atlantic). Sources of these waters may have been the low-latitude eastern Tethys and high-latitude North Atlantic. Changes in intermediate to deep water sources were probably steered by eccentricity-controlled insolation fluctuations. Lower insolation favored the formation of high-latitude deep waters due to positive feedback mechanisms resulting in high-latitude cooling. This led to a displacement of low-latitude deep waters at Blake Nose. Higher insolation reduced intermediate to deep-water formation in high latitudes, yielding a more northern flow of low-latitude deep waters.

A high resolution stable isotope record of the penultimate deglaciation in lake sediments below the city of Amsterdam, The Netherlands

A detailed record of the deglaciation history of the penultimate glacial to interglacial transition is given, mainly based on the stable isotopes of calcium carbonate-containing lake sediments in a glacially excavated depression below the city of Amsterdam. Initially, this depression, the Amsterdam Basin, formed part of a lake covering the western and central part of the Netherlands and extending for an unknown distance into the present North Sea. Annual layer counting shows that the lake drained after about a millennium, leaving shallow pools in the remaining depressions. The latter changed again into larger and deeper lakes including the Holland Lake during the rise of sea level in the early Eemian. Oxygen isotope values of the early lake sediments indicate an interglacial summer climate, but the nearness of dead-ice fields caused severe winters. The isotope record is furthermore characterized by the influx of large amounts of isotopically light water supplied by the river Rhine. A change to much colder conditions occurred simultaneously with the draining of the Holland Lake, as appears from oxygen isotope values and the sudden increase in non-arboreal pollen. This interval is correlated with the Kattegat Stadial and the sea-level standstill of Aladdins Cave on the Huon Peninsula of New Guinea. A short climate wiggle occurs at the end of this interval. The onset of the Eemian is marked by a rapid warming of ∼5°C which extends into local pollen-zone E3. Provided that our correlation of this cold interval with the sea-level standstill of Aladdins Cave is correct, the time interval between the earliest lake sediments in the Amsterdam Basin and the Eemian highstand took about 5–6 millennia.