| Title: | The Cretaceous marine osmium (Os) isotope record |
| Author: | Greg Ravizza |
| Date Submitted: | 04/29/2002 |
| Address: | MS #22
Woods Hole
MA
USA
02543
|
| Phone: | 508 457-2841 |
| Email: | gravizza@whoi.edu |
| Co-Authors: | Peucker-Ehrenbrink, Bernhard,WHOI, behrenbrink@whoi.edu; Blusztajn, Jurek, WHOI, jblusztajn@whoi.edu; Abbruzzese, Tracy, WHOI, tabbruzzese@whoi.edu |
| Affiliation: | Woods Hole Oceanographic Institution |
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| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725514 |
| Keywords: | LIPS radiogenic isotopes, chemostratigraphy, osmium |
| Abstract: | Relative to the Cenozoic, the marine Os isotope record of the Cretaceous Period is very poorly documented. The purpose of this abstract is two-fold First, to summarize the available data constraining seawater Os isotope variations during the Cretaceous. Second, to suggest how higher resolution records of Cretaceous seawater Os isotope variations might contribute to broader efforts to better understand the causative factors responsible for Cretaceous climatic extremes.
Os isotope analyses of Cretaceous marine sediments are few and far between, never the less it is unambiguous that this Period is characterized by large amplitude variations in seawater 187Os/188Os. Data coverage is best during the latest Cretaceous where low resolution temporal resolution analyses from several pelagic clay sequences yield a coherent picture of seawater Os isotope variations. There is a pronounced minimum at the K-T boundary (187Os/188Os as low as 0.168), preceded by higher 187Os/188Os ratios between 66 and 80 Ma, as high 0.7. Based on analyses of metalliferous sediments from Cyprus and Oman, there is an excursion to low 187Os/188Os ( = 0.5) close to the Cenomanian-Turonian bondary (CBT) from ratios as high as 0.68 in the early Cenomanian. By the late Coniacian 187Os/188Os recovered to 0.58. To our knowledge there are no data constraining the structure of the Cretaceous portion of the marine Os isotope record prior to the Cenomanian.
Although the data described above are sparsely distributed in time, we suggest two potential applications of the Cretaceous portion of the marine Os isotope record. First we hypothesize that large amplitude seawater 187Os/188Os variations might be exploited in a manner similar to the marine Sr isotope record. In the latest Cretaceous the rapid decline in seawater 187Os/188Os from 0.7 to 0.4 over the last several million years of the Cretaceous may provide improved age estimates between magnetic reversal datums. Os isotope stratigraphy also may be a useful complement to Sr isotope stratigraphy within the Cretaceous quite zone.
There is a long standing hypothesis that "superplumes" played a causative role in mid-Cretaceous warmth, anoxia and enhanced carbon burial. The local 187Os/188Os minimum near the CTB (described above) is analogous to the well documented local minimum in seawater 87Sr/86Sr near the CTB, with both isotope records indicating relatively greater mantle influence on ocean chemistry. However, the dip in seawater 87Sr/86Sr spans a time interval that is too broad to investigate the relative timing of various phenomena in detail. In essence, the response time of the marine Sr isotope system is too slow to address these issues effectively. We hypothesize that the short marine residence time of Os will allow the marine 187Os/188Os record to provide a record of LIP emplacement history that is more highly resolved in time than that provided by the marine 87Sr/86Sr record. With this improved temporal resolution we believe it will be possible to place new constraints on the time scale and nature of LIP volcanism, and also to better correlate this record with data from the sediment record that constrain ocean anoxia and carbon burial rates. |
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