GEOCHEMISTRY OF CENOMANIAN DEPOSITS FROM THE WESTERN TETHYS AT DSDP SITE 386 AND 144: PALEOCEANOGRAPHIC AND TECTONIC IMPLICATIONS

          High-resolution stratigraphic analyses, including Total Organic (TOC) and Total Inorganic Carbon (TIC), organic-carbon-isotopes (d¹³C_org), and minor elements, were carried out on Cenomanian sediments from DSDP Site 386 (Bermuda Rise; 4,782 meters) and Site 144 (Demerara Rise; 2,957 meters). The results reveal a relationship related to existing paleoclimates, modulated by paleophysiography, that controlled paleodepths and paleoenvironmental conditions.

          Sediments at Site 386 show high-frequency varicolored layers alternating from light tan to various shades of red, green, gray, and black. TIC ranges from 0% to 67%, and averages 2%. TOC ranges from 0% to at least 16.6%, and averages 1.4%. Clay content, expressed as relative Si wt%, ranges from 4.5% to 44%, with an average of 29%. d¹³C_org values range between –27‰ and –23‰, and the interval studied shows no apparent expression of the well-documented carbon-isotope ‘spike' of the Cenomanian-Turonian boundary. The mid-Cenomanian double peak, however, seems to be present.

          Sediments at Site 144 are olive gray to black throughout, with TIC consistently higher than at Site 386, ranging from 15% to 84%, with an average of 44%. Similarly, TOC ranges higher, from 3% to 14%, and averages 9.6%. Clay content ranges from 3% to 12.5%, and averages 6%. d¹³C_org values range between –28‰ and –27‰, although carbon-isotopes display neither the Cenomanian-Turonian boundary ‘spike' nor the mid-Cenomanian double peak.

          Patterns of variations of the different parameters imply that recurrent paleoceanographic conditions suggestive of climatically-induced fluctuations favored frequent preservation of TIC and TOC at Site 144, but only occasionally at Site 386. Differences in facies between the two sites must reflect their respective paleobathymetry, and variations in TIC/TOC recorded at Site 386 are interpreted to represent fluctuations that affected local paleoproductivity as well as the CCD level. We surmise that elemental results indicative of high clay contents at Site 386 can be attributed to volcanic influences, but not at Site 144. Average d¹³C_org values for both sites tend to be isotopically lighter than those of Recent samples, and thus are compatible with earlier works that concluded CO₂ concentrations were higher during this time period due to tectonovolcanic events.