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Publication Title | MIOCENE TO EARLY PLIOCENE OXYGEN AND CARBON ISOTOPE STRATIGRAPHY IN THE SOUTHWEST PACIFIC, DEEP SEA DRILLING PROJECT LEG 901

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42. MIOCENE TO EARLY PLIOCENE OXYGEN AND CARBON ISOTOPE STRATIGRAPHY IN THE SOUTHWEST PACIFIC, DEEP SEA DRILLING PROJECT LEG 901

James P. Kennett, Graduate School of Oceanography, University of Rhode Island

ABSTRACT

High resolution oxygen and carbon isotope stratigraphy is presented for Miocene to early Pliocene sequences at three DSDP sites from the Lord Howe Rise, southwest Pacific, at water depths ranging from 1,300 to 2,000 m. Site 588 is located in the warm subtropics (~ 26°S), whereas Sites 590 and 591 are positioned in transitional (northerntemperate) water masses (~ 31°S). Benthic foraminiferal oxygen and carbon isotope analyses were conducted on all sites; plankton ic foraminiferal isotope data were generated for Site 590 only. Sample resolution in these sequences is on the order of 50,000 yr. or better. The chronological framework employed in this study is based largely upon ages assigned to Neo gene calcareous nannoplankton boundaries.

The benthic oxygen isotope record exhibits several major features during the Neogene. During most of the early Mi ocene, 1 8 values were relatively low, reaching minimum values in the late early Miocene (19.5 to 16.5 Ma), and record ing the climax of Neogene warmth. This was followed by a major increase in benthic 1 8 values between ~ 16.5 and 13.5 Ma, which is interpreted as representing major, permanent accumulation of the East Antarctic ice sheet and cool ing of bottom waters. During the 3 m.y. 1 8 O enrichment, surface waters at these middle latitudes warmed between 16 and 14.5 Ma.

During the remainder of the middle and late Miocene, benthic 1 8 values exhibit distinct fluctuations, but the aver age value remained unchanged. The isotopic data show two distinct episodes of climatic cooling close to the middle/late Miocene boundary. The earliest of these events occurred between 12.5 and 11.5 Ma in the latest middle Miocene. The second cooling event occurred from 11 to 9 Ma, and is marked by some of the highest 1 8 values of the entireMiocene. This was followed by relative warmth during the middle part of the late Miocene. The latest Miocene and earliest Plio cene (6.2 to 4.5 Ma) were marked by relatively high 1 8 values, indicating increased cooling and glaciation.

During the middle Pliocene, at about 3.4 Ma, a 0.4‰ increase in benthic 1 8 documents a net increase in average global ice volume and cooling of bottom waters. During this interval of increased glaciation, surface waters warmed by 2 3°C in southern middle latitude regions. During the late Pliocene, between 2.6 and 2.4 Ma, a further increase in 1 8 occurred; this has been interpreted by previous workers as heralding the onset of Northern Hemisphere glaciation.

Surface water warming in the middle latitudes occurred in association with major high latitude glacial increases in the early middle Miocene (16 14 Ma), middle Pliocene ( 3.5 Ma), and late Pliocene (~2.4 Ma). These intervals were also marked by increases in the vertical temperature gradient in the open ocean.

Intersite correlation is enhanced by using carbon isotope stratigraphy. The great similarity of the 13C time series re cords within and between ocean basins and with water depth clearly indicates that changes in oceanwide average 13C of HCOJ in seawater dominated the records, rather than local effects. Broad changes in the Neogene 13C record were caused largely by transfer of organic carbon between continental and oceanic reservoirs. These transfers were caused by marine transgressions and regressions on the continental margins.

The dominant feature of Neogene 1 3 C stratigraphy is a broad late early to early middle Miocene increase of about l‰ between ~ 19 and 14.5 Ma. This trend occurred contemporaneously with a period of maximum coastal onlap (transgression) and maximum Neogene climatic warmth. The 13C trend terminated during the expansion of the Ant arctic ice sheet and associated marine regression.

The latest Miocene carbon isotope shift (of up to O.75‰) at 6.2 Ma is clearly recorded in all sites examined and was followed by relatively low values during the remainder of the Neogene. This shift was caused by a glacioeustatic sea level lowering that exposed continental margins via regression and ultimately increased the flux of organic carbon to the deep sea. An increase in 1 3 C values during the early Pliocene (~ 5 to 4 Ma) resulted from marine transgression during a time of global warmth.

INTRODUCTION

Cenozoic cooling, which began in the middle Eocene, proceeded gradually, but was punctuated by sudden tran sitions from one climatic state to another. Global cli mate underwent major changes during the Miocene Ep och. The most important change occurred during the early middle Miocene, between 1 7 and 14 Ma. A marked increase in 1 8 of benthic foraminiferal calcite at this time is commonly interpreted as recording the

Kennett, J. P., von der Borch, C. C , et al., Init. Repts. DSDP, 90: Washington (U.S. Govt. Printing Office).

2 Address: Graduate School of Oceanography, University of Rhode Island, Narragan sett, R.I. 92882 1197.

rapid growth of the East Antarctic ice sheet and associ ated cooling of high latitude surface and deep ocean wa ters (Savin et al., 1975; Shackleton and Kennett, 1975b; Kennett, 1977; Schnitker, 1980; Keigwin, 1979; Wood ruff et al., 1981; Savin et al., 1981). Others have inter preted this increase in 1 8 values as reflecting only a decrease in bottom water temperatures, unaccompanied by any increase in Antarctic ice (Matthews and Poore, 1980).

Oxygen isotope ratios of benthic and planktonic for aminifers indicate that these events were accompanied by a marked increase in the equator to pole thermal gra dient and an associated decrease in meridional heat trans port (Savin et al., 1975, in press). Tropical Pacific near surface temperatures increased while high latitude regions

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