Margot H. Saher

Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain)

ABSTRACT The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian-Moscovian platform-to-basin transect, 13C and 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in 13C and 18O signatures from platform top (lower values) to basin (higher values). In the case of 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

Sea surface temperatures of the western Arabian Sea during the last deglaciation

[1] In this study we present a sea surface temperature (SST) record from the western Arabian Sea for the last 20,000 years. We produced centennial-scale d 18 O and Mg/Ca SST time series of core NIOP929 with focus on the glacial-interglacial transition. The western Arabian Sea is influenced by the seasonal NE and SW monsoon wind systems. Lowest SSTs occur during the SW monsoon season because of upwelling of cold water, and highest SSTs can be found in the low-productivity intermonsoon season. The Mg/Ca-based temperature record reflects the integrated SST of the SW and NE monsoon seasons. The results show a glacial-interglacial SST difference of � 2C, which is corroborated by findings from other Arabian Sea cores. At 19 ka B.P. a yet undescribed warm event of several hundred years duration is found, which is also reflected in the d 18 O record. A second centennial-scale high SST/low d 18 O event is observed at 17 ka B.P. This event forms the onset of the stepwise yet persistent trend toward Holocene temperatures. Highest Mg/Ca-derived SSTs in the NIOP929 record occurred between 13 and 10 ka B.P. Interglacial SST is � 24C, indicating influence of upwelling. The onset of Arabian Sea warming occurs when the North Atlantic is experiencing minimum temperatures. The rapid temperature variations at 19, 17, and 13 ka B.P. are difficult to explain with monsoon changes alone and are most likely also linked to regional hydrographic changes, such as trade wind induced variations in warm water advection.

Western Arabian Sea SST during the penultimate interglacial: A comparison of U37K′ and Mg/Ca paleothermometry

Millennial-scale records of planktonic foraminiferal Mg/Ca, bulk sediment U37 K′, and planktonic foraminiferal δ 18O are presented across the last two deglaciations in sediment core NIOP929 from the Arabian Sea. Mg/Ca-derived temperature variability during the penultimate and last deglacial periods falls within the range of modern day Arabian Sea temperatures, which are influenced by monsoon-driven upwelling. The U37 K′-derived temperatures in MIS 5e are similar to modern intermonsoon values and are on average 3.5°C higher than the Mg/Ca temperatures in the same period. MIS 5e U37 K′ and Mg/Ca temperatures are 1.5°C warmer than during the Holocene, while the U37 K′-Mg/Ca temperature difference was about twice as large during MIS 5e. This is surprising as, nowadays, both proxy carriers have a very similar seasonal and depth distribution. Partial explanations for the MIS 5e U37 K′-Mg/Ca temperature offset include carbonate dissolution, the change in dominant alkenone-producing species, and possibly lateral advection of alkenone-bearing material and a change in seasonal or depth distribution of proxy carriers. Our findings suggest that (1) Mg/Ca of G. ruber documents seawater temperature in the same way during both studied deglaciations as in the present, with respect to, e.g., season and depth, and (2) U37 K′-based temperatures from MIS 5 (or older) represent neither upwelling SST nor annual average SST (as it does in the present and the Holocene) but a higher temperature, despite alkenone production mainly occurring in the upwelling season. Further we report that at the onset of the deglacial warming, the Mg/Ca record leads the U37 K′ record by 4 ka, of which a maximum of 2 ka may be explained by postdepositional processes. Deglacial warming in both temperature records leads the deglacial decrease in the δ 18O profile, and Mg/Ca-based temperature returns to lower values before δ 18O has reached minimum interglacial values. This indicates a substantial lead in Arabian Sea warming relative to global ice melting.

Belemnite-based strontium, carbon and oxygen isotope stratigraphy of the type area of the Maastrichtian Stage *

Belemnitellid cephalopods from the Maastrichtian stratotype area (southeast Netherlands) are shown to be comparatively well preserved. Although partial diagenetic alteration has been observed, micromilling techniques have permitted the extraction of pristine belemnite calcite, suitable for the reconstruction of strontium (Sr), oxygen (O) and carbon (C) isotope variation of Maastrichtian seawater. A distinct Sr isotope pattern in the Maastricht record can be matched stratigraphically with records from Hemmoor (northern Germany), El Kef (Tunisia) and ODP site 690 (Maud Rise, Antarctica), leading to a new chemostratigraphical age model for the Maastrichtian stratotype section. Our data improve currently applied strontium isotope stratigraphical reference curves by revealing an Sr isotope inflection pattern near the lower/upper Maastrichtian boundary that is a potentially diagnostic feature for intra-Maastrichtian stratigraphical correlation between distant sections. Belemnites further show significant stratigraphical oxygen isotope variation through the Maastrichtian. We interpret this variation to have resulted from palaeoceanographic reorganisations in the Atlantic Ocean during this time interval.

Testate amoebae as sea-level indicators in Northwestern Norway: developments in sample preparation and analysis

Proxy based sea-level reconstructions are an important tool for defining past patterns of sea-level change and salt-marsh testate amoebae are a newly emerging proxy with high potential as sea-level indicators. This study develops existing analytical techniques con- cerned with the preparation and counting of testate amoebae for sea-level studies and demonstrates the predictive power of this group of micro-organisms. Two salt marshes in northwestern Norway were sampled for testate amoebae and multiple sub-samples were prepared using different procedures and count totals. Analytical efficiency can be improved upon by using a mild alkali, chemical disaggregant (5% KOH) to break up fibrous, salt-marsh peat and concentrate tests prior to counting. A count total of 100 individuals, rather than 150, can be used to make time gains with little or no loss of taxon information. The Norwegian salt-marsh testate amoebae showed strong zonation relative to tidal elevation. Key indicator taxa from the high marsh included Centropyxis cassis type, Cyclopyxis arcelloides type and Eugly- pha spp. Those from the low marsh included Difflugia pristis type and a distinctive morphotype of Centropyxis platystoma type. Combined, the two surface data sets from Norway were capable of predicting marsh surface elevations to within ± 0.09 m.