Preservation of delta 18 O and delta 13 C in belemnite rostra from the Jurassic/Early Cretaceous successions

Abstract

Stratigraphic collection of 263 belemnite rostra (guards) from Jurassic to Lower Cretaceous sediments in the Moscow area of Russia, northern Germany, England, New Zealand, and Morocco were analyzed to determine temporal variations in their delta 18 O and delta 13 C as a proxy for isotopic composition of surface seawater. Internal structures of the rostra were studied using cathodoluminescence and SEM techniques, their trace element (Fe, Mn, Mg, and Sr), and the stable carbon and oxygen isotope compositions. The average stratigraphic resolution is about 1 Ma. These samples represent a broad range of paleolatitudes and paleoceanographic settings, ranging from 42 degrees N to 82 degrees S, covering boreal to subtropical, brackish to open marine shelf paleoenvironments. The rostra originally contained domains enriched in organic matter that were later replaced by luminescent diagenetic calcite, thereby generating an impression of seasonal growth rings. Diagenetic calcite accounts for up to 6.8 percent of the rostrum. The trace element content of the rostra range from 227 to 3935 ppm for Mg, 379 to 1514 ppm for Sr, and up to 305 and 3662 ppm for Mn and Fe, respectively. However, selective drilling of only primary calcitic domains resulted in 90 percent of all analyzed samples to fall within the range of trace element values typical for modern low Mg-calcite shells. The chemical and textural data suggest that the primary calcite of the rostra is well preserved, as is its isotopic signature. The delta 13 C and delta 18 O measurements define oscillating temporal bands, of about 3 to 4 per mil width, around more or less modern marine values of +2 and -1 per mil, respectively. Oxygen isotopes in the well preserved rostra most likely reflect temperature variations within a approximately 14+ or -8 degrees C range. The observed 3 to 4 per mil spread of values between contemporaneous specimens, which is comparable to the scatter of data reported for modern low-Mg calcitic brachiopod shells, probably reflects the diversity of local ecosystems, although some of the spread may be due to real short term temporal variations within single biozones. The delta 13 C values of the rostra define a band similar to that of the oxygen isotopes, and the temporal oscillations of both isotope curves are commonly in phase. The above dataset, complemented by the results of Jones, Jenkyns, and Hesselbo (1994) and Jones and others (1994), yields a composite delta 18 O and delta 13 C variations in belemnite guards for the entire Jurassic-Early Cretaceous interval. For delta 18 O, the oscillating band indicates that the most negative values occur at times of independently postulated global and/or regional anoxic events, such as those in the Toarcian and Kimmeridgian as well as the Aptian. These OAEs appear to be characterized by up to 10 degrees C higher seawater temperatures than the intervening background times. The delta 13 C record shows 13 C depletion for the Kimmeridgian event but enrichment for the other two, suggesting that the delta 13 C temporal oscillations and OAEs are not simply a reflection of surficial productivity due to operation of the biological pump, but rather of changing oceanic circulation patterns due to variations in latitudinal surface temperature gradients.