Astronomically tuned Plio–Pleistocene benthic δ¹⁸O record from South China Sea and Atlantic–Pacific comparison

Abstract

Based on benthic foraminiferal δ¹⁸O from ODP Site 1143, a 5-Myr astronomical timescale for the West Pacific Plio–Pleistocene was established using an automatic orbital tuning method. The tuned Brunhes/Matuyama paleomagnetic polarity reversal age agrees well with the previously published age of 0.78 Ma. The tuned ages for several planktonic foraminifer bio-events also agree well with published dates, and new ages for some other bio-events in the South China Sea were also estimated. The benthic δ¹⁸O from Site 1143 is highly coherent with the Earth’s orbit (ETP) both at the obliquity and precession bands for the last 5 Myr, and at the eccentricity band for the last 2 Myr. In general, the 41-kyr cycle was dominant through the Plio–Pleistocene although the 23-kyr cycle was also very strong. The 100-kyr cycle became dominant only during the last 1 Myr. A comparison of the benthic δ¹⁸O between the Atlantic (ODP 659) and the East and West Pacific (846 and 1143) reveals that the Atlantic–Pacific benthic oxygen isotope difference ratio (Δδ¹⁸O_Atl-Pac) displays an increasing trend in three time intervals: 3.6–2.7 Ma, 2.7–2.1 Ma and 1.5–0.25 Ma. Each of the intervals begins with a rapid negative shift in Δδ¹⁸O_Atl-Pac, followed by a long period with an increasing trend, corresponding to the growth of the Northern Hemisphere ice sheet. This means that all three intervals of ice sheet growth in the Northern Hemisphere were accompanied at the beginning by a rapid relative warming of deep water in the Atlantic as compared to that of the Pacific, followed by its gradual relative cooling. This general trend, superimposed on the frequent fluctuations with glacial cycles, should yield insights into the processes leading to the boreal glaciation. Cross-spectral analyses of the Δδ¹⁸O_Atl-Pac with the Earth’s orbit suggests that after the initiation of Northern Hemisphere glaciation at about 2.5 Ma, obliquity rather than precession had become the dominant force controlling the vertical structure or thermohaline circulation in the paleo-ocean.

Introduction

The last 5 Myr of paleoceanographic history is crucial to our understanding of the Earth’s climate system. The final closure of the Panama Isthmus and the Indonesian seaway, further uplift of the Tibetan Plateau and the onset of major ice sheets in the Northern Hemisphere, a series of major events leading to the modern patterns of the Earth system, are located within this period of time. Deep sea sediments are the main carrier of high-resolution long-term records of Plio–Pleistocene ocean history, yet adequate isotopic sequences are rare in the literature. Up to now, only a few ODP sites provided continuous isotopic records of benthic foraminifera over the last 5 Myr with time resolution at the millennial scale. Of those, two are from the Eastern Pacific (ODP Sites 846 [1], [2] and 849 [3]) and one from the Atlantic (ODP Site 659 [4]) (Table 1). Because of the great water depths and poor carbonate preservation, no long sequence is available from the Western Pacific, despite its undoubted significance.

The hemipelagic sediments from the South China Sea (SCS) are distinguished by high sedimentation rate and carbonate preservation. With the modern carbonate compensation depth at about 3500 m, the SCS offers a unique opportunity to collect from the region a high-resolution oxygen isotope record covering a long period of time. ODP Site 1143 from the SCS provides for the first time a high-resolution long-term isotopic record in the Western Pacific. As seen from Table 1, this is the only site in any ocean which has yielded complete records of benthic and planktonic isotope data covering the entire last 5 Myr.