Showing papers by "Kyger C. Lohmann published in 1991"
TL;DR: In this article, a micro-sampling of late Devonian abiotic marine calcite has been performed for 87 Sr 86 Sr ratios, δ18O and δ13C values, and minor element concentrations, and it has been shown that marine cement crystals from the Alberta and Canning Basins have escaped diagenetic alteration and preserve original marine δ 18O values (−4.8% ± 0.5, PDB), δ 13C values (+2.0 to +3.0%), which are best explained by precipitation from 18O-
152 citations
TL;DR: Sr isotope data from two boreholes within the lagoon at Enewetak Atoll have been used to evaluate the use of such data to correlate, date, and monitor sea level change in shallow water carbonate sequences as discussed by the authors.
Abstract: Sr isotope data from two boreholes within the lagoon at Enewetak Atoll have been used to evaluate the use of such data to correlate, date, and monitor sea level change in shallow water carbonate sequences. Correlative stratigraphic intervals of relatively invariant δ87Sr, separated by abrupt transitions to lower δ87Sr with increasing depth, are recognized in both boreholes. Conversion of δ87Sr values to age via calibration with the seawater δ87Sr trend with age indicates that correlative and synchronous deposition of atoll sediments occurred at ∼ 0.4, 1.2, and 2.1 Ma. In contrast, a ∼5 m.y. hiatus is recognized in one borehole but not the other. Sr isotope stratigraphy (SIS) is a powerful stratigraphic and chronostratigraphic tool in shallow water carbonate sequences only when significant secular variation of δ87Sr occurs and retention of depositional δ87Sr values is demonstrated. The latter is best demonstrated when δ87Sr data, are integrated with δ18O, δ13C, Sr content data and petrographic observations. Several diagenetically altered intervals have greater δ87Sr values, low δ13C values, and low Sr/Ca ratios relative to adjacent intervals, a combination that is consistent with open-system meteoric diagenesis. Calcite cements from these intervals have early Pleistocene (∼1.2 Ma) δ87Sr values despite their occurrence well within the late Pliocene (∼2.1 Ma) sequence. Thus local sedimentological and diagenetic processes have produced intralagoon variability in the SIS of the two boreholes, complicating subsurface stratigraphic correlations. The occurrence of anomalously young calcite cement relative to adjacent limestone is a direct response of the interaction of sea level change and meteoric phreatic diagenesis whereby overlying metastable carbonates, with greater δ87Sr values, are dissolved during periods of atoll emergence and sea level lowstand liberating Sr and soil-gas CO2 to the pore fluid, which is then incorporated into downflow meteoric phreatic diagenetic calcite. The stratigraphic relation between the sites of carbonate dissolution and diagenetic calcite precipitation suggest an early Pleistocene (∼1.2 Ma) sea level fall of between 34 and 64 m. Moreover, the stratigraphic position and inferred age of paleophreatic lens calcite when backtracked for subsidence suggest that early Pleistocene (∼1.2 Ma) sea level was ∼72–81 m (range of 60–100 m) below present sea level. The Enewetak record of early Pleistocene sea level lowstand elevation agrees reasonably well with proxy sea level records inferred from sequence stratigraphy and deep-sea foraminiferal δ18O data. However, the Enewetak record of the amplitude of early Pleistocene sea level change is more consistent with the deep-sea foraminiferal δ18O proxy than the sequence stratigraphy proxy.
42 citations
01 Jan 1991
TL;DR: Oxygen and carbon isotope records have been developed for the Cenozoic carbonate oozes of Sites 752, 754, 756, and 757 based on the analysis of monospecific benthic foraminifers as discussed by the authors.
Abstract: Oxygen and carbon isotopic records have been developed for the Cenozoic carbonate oozes of Sites 752, 754, 756, and 757 based on the analysis of monospecific benthic foraminifers. The intent of this report is to provide a basic isotopic stratigraphy for use in other paleoceanographic studies. The oxygen isotope record displays the enrichments associated with cooling or ice volume buildup at the Eocene/Oligocene boundary, in the middle Miocene, and in the upper Pliocene. The carbon isotopic record contains the Chron 16 enrichment in the lower Miocene and the Chron 6 depletion in the uppermost Miocene.
11 citations