John W Farrell

Improved chronostratigraphic reference curve of late Neogene seawater 87Sr/86Sr

We present a reference curve of seawater 87 Sr/ 86 Sr variation through the past 7 m.y. based on 455 samples of planktonic foraminifera from Ocean Drilling Program (ODP) Site 758 in the Indian Ocean. This single-site curve is superior to extant curves because of several features: (1) a continuous sedimentary section free of disturbances; (2) a well-determined and precise chronostratigraphy that is calibrated to numeric age on the basis of continuous magnetostratigraphy and a refined astronomical time scale; (3) a high temporal resolution (sample interval averages 15 ka); (4) an improved analytical reproducibility; and (5) a uniformly high sample quality. The gross structure of the Site 758 curve is generally similar to that observed in previous work, but the increased temporal resolution and decreased analytical scatter of the curve provide improved chronostratigraphic resolution. At the 95% confidence level, uncertainties on age estimates range from ±0.60 m.y. in the lower Pleistocene and upper Miocene to ±2.03 m.y. in the middle Pliocene. Moreover, this detailed record identifies two periods of steplike increases in seawater 87 Sr/ 86 Sr (6.10 to 5.80 Ma and 1.46 to 1.13 Ma) that are characterized by rates of change of ∼150 × 10 −6 /m.y. These features, if verified at other sites, indicate abrupt changes in the Sr mass balance of the ocean and may be used as precise event markers in late Neogene marine sections.

Pacific CaCO3 Preservation and δ18O Since 4 Ma: Paleoceanic and Paleoclimatic Implications

The Pliocene-Pleistocene history of CaCO3 preservation in the central equatorial Pacific is reconstructed from a suite of deep-sea cores and is compared to fluctuations in global ice volume inferred from δ18O records. The results are highlighted by: (1) a strong covariation between CaCO3 preservation and ice volume over 104 to 106 year time scales; (2) a long-term increase in ice volume and CaCO3 preservation since 3.9 Ma demonstrated by a deepening of the lysocline and the carbonate critical depth; (3) a dramatic shift to greater CaCO3 preservation at 2.9 Ma; (4) distinctive ice-volume growth and CaCO3 preservation events at 2.4 Ma, which are associated with the significant intensification of northern hemisphere glaciation; (5) a mid-Pleistocene transition to 100-kyr cyclicity in both CaCO3 preservation and ice volume; and (6) a 600-kyr Brunhes dissolution cycle superimposed on the late Pleistocene glacial/interglacial 100-kyr cycles. CaCO3 preservation primarily reflects the carbonate chemistry of abyssal waters and is controlled by long-term (106 year) and short-term (104 to 105 year) biogeochemical cycling and by distinct paleoclimatic events. We attribute the long-term increase in CaCO3 preservation primarily to a fractionation of CaCO3 deposition from continental shelf to ocean basin, and secondarily to a gradual rise in the riverine and glaciofluvial flux of Ca++. On shorter time scales, the fluctuations in CaCO3 preservation slightly lag ice volume fluctuations and are attributed to climatically induced changes in the circulation and chemistry of Pacific deep water.

Timescale and paleoceanographic implications of a 3.6 m.y. oxygen isotope record from the northeast Indian Ocean (Ocean Drilling Program Site 758)

Numerous studies have shown that δ18O records from benthic and planktonic foraminifera, primarily a proxy of global ice volume variations, reflect Milankovitch periodicities. To study climatic response to orbital forcing at Ocean Drilling Program site 758, we have generated continuous δ18O and δ13C records from a single benthic foraminiferal species Cibicides wuellerstorfi for the last 3.6 m.y. and extended the planktonic foraminiferal isotope records of Farrell and Janecek (1991) (0-2.5 Ma, based on Globigerinoides sacculifer) to 3.6 Ma (Chen, 1994). We then constructed an age model by matching, correlating and tuning the benthic δ18O record to a model simulation of ice volume (Imbrie and Imbrie, 1980). The filtered 41- and 23-kyr signals based on the resultant astronomically tuned age model are highly correlated to obliquity (r=0.83) and precession (r=0.75), respectively. Although derived with methodology different from Shackleton et al. (1990) and Hilgen (1991a, b), our results generally agree with their published astronomical timescales for the time interval from 0 to 3.0 Ma, providing additional support for the newly emerging chronology based on orbital tuning. Slight discrepancies exist in the time interval from 3.0 to 3.6 Ma, suggesting several possibilities, including differences in the approaches of orbital tuning and the relatively low amplitude of δ18O variations in our record. However, even if the discrepancies are due to the relatively low amplitude of the isotope signals in our record at 3.0–3.6 Ma, our resultant timescale as a whole does not adversely affect our evaluation of the paleoclimatology and paleoceanography of the Indian Ocean, such as the evolution of the 100-, 41- and 23-kyr cycles, and variation of global ice volume and deepwater temperature during the past 3.6 m.y.

Paleoceanography of the eastern Indian Ocean from ODP Leg 121 drilling on Broken Ridge

Broken Ridge, in the eastern Indian Ocean,is overlain by about 1,600 m of middle Cretaceous to Pleistocene tuffaceous and carbonate sediments that record the oceanographic history of southern hemisphere mid-to high-latitude regions. Prior to about 42 Ma, Broken Ridge formed the northern part of the broad Kerguelen-Broken Ridge Plateau. During the middle Eocene, this feature was split by the newly forming Southeast Indian Ocean Ridge; since then, Broken Ridge has drifted north from about 55° to 31°S. The lower part of the sedimentary section is characterized by Turonian to Santonian tuffs that contain abundant glauconite and some carbonate. The tuffs record a large but apparently local volcanic input that characterized the central part of Broken Ridge into the early Tertiary. Maestrichtian shallow-water(several hundred to 1,000 m depth) limestones and cherts accumulated at some of the highest rates ever documented from the open ocean, 4 to 5 g (cm 2 10 3 yr) -1 . A complete (with all biostratigraphic zones) Cretaceous-Tertiary boundary section was recovered from site 752. The first 1.5 m.y. of the Tertiary is characterized by an order-of-magnitude reduction in the flux of biogenic sediments, indicating a period of sharply reduced biological productivity at 55°S, following which the carbonate and silica sedimentation rates almost reach the previous high values of the latest Cretaceous. We recovered a complete section through the Paleocene that contains all major fossil groups and is more than 300 m thick, perhaps the best pelagic Paleocene section encountered in ocean drilling. About 42 Ma, Broken Ridge was uplifted 2,500 m in response to the intra-plateau rifting event; subsequent erosion and deposition has resulted in a prominent Eocene angular unconformity atop the ridge. An Oligocene disconformity characterized by a widespread pebble layer probably represents the 30 Ma sea-level fall. The Neogene pelagic ooze on Broken Ridge has been winnowed, and thus its grain size provides a direct physical record of the energy of the southern hemisphere drift current in the Indian Ocean for the past 30 m.y.

Improved chronostratigraphic reference curve of late Neogene seawater 87Sr/86 Sr: Comment and Reply

NBS 987 averaged 0.710241 (09) (n 5 39) during the period of these analyses. Errors on 87 Sr/ 86 Sr are given as 2s(95%) in the last two digits. 2 The Sr analyses are normalized to 86 Sr/ 88 Sr 5 0.11940. Analyses of NBS 987 averaged 0.710230 (11) during the period of these analyses. Errors on 87 Sr/ 86 Sr are given as 2s(95%) in the last two digits.