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We present astronomically tuned high time resolution (3–4 kyr) benthic foraminiferal δ 18 O and δ 13 C records between 9 Ma and 5 Ma from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS). We found that the late Miocene ocean carbon shift (LMOCS) which was linked with the late Miocene/early Pliocene “Biogenic Bloom” also occurred in the northern and southern SCS. A marked deep-water δ 13 C gradient as much as ~ 1.0‰ of the northern and southern SCS was observed between 6.5 Ma and 5.9 Ma, which was probably caused by the significant difference in paleoproductivity between the northern and southern SCS. The deep δ 13 C gradient was likely produced by rapid tectonic subsidence in the northern SCS at ~ 6.0 Ma and the final formation of the Bashi Strait at ~ 6.5 Ma. A reversed benthic δ 13 C gradient of the northern and southern SCS was also observed between 1.2 Ma and 0.35 Ma, which was probably caused by the consecutive closures of the interarc water gates within the Bashi strait at ~ 3.0 Ma and ~ 1.2 Ma. Closures of the interarc water gates within the Bashi strait at ~ 3.0 Ma and ~ 1.2 Ma further semi-enclosed the SCS basin and made the paleoproductivity the major factor influencing the deep water δ 13 C, leading to the reversed deep water δ 13 C gradient of the northern and southern SCS between 1.2 Ma and 0.35 Ma. [ABSTRACT FROM AUTHOR]

The lack of monthly-resolved paleoenvironmental records beyond the instrumental era hinders our full understanding of global climate dynamics. Tridacna gigas shells have a large potential to quantitatively document the variability of the ocean surface environment on time scales from daily to interannual in the past, while the applicability of the geochemical proxies of Tridacna gigas needs to be quantitively investigated. Here we establish a chronology for a Tridacna gigas shell from the southern South China Sea (SCS) by identifying daily growth laminae using a laser scanning confocal microscope (LSCM). Our high-resolution Tridacna gigas shell δ18O shell record throughout 1989–2013 shows a high correlation with changes in local precipitation and the index of El Niño-Southern Oscillations (ENSO) on the interannual time scale. Together with observed sea surface temperature (SST), we calculated seawater δ18O (δ18O sw) and derived sea surface salinity (SSS) change in the southern SCS based on the Tridacna gigas shell δ18O record. Our salinity-budget calculation results suggest that the SSS variability in the southern SCS is predominantly affected by the vertical mixing and sea surface freshwater flux. Our study reveals that the Tridacna gigas shell is an ideal archive for high-resolution and quantitative paleoenvironmental reconstructions in tropical ocean regions, thus providing a new window to explore paleoenvironmental changes on annual and interannual time scales. • A method to analyze the growth chronology of Tridacna gigas bivalve shell was developed. • A δ18O record of Tridacna gigas shell between 1989 and 2013 was generated. • The δ18O record indicates the variability of precipitation and ENSO. • A sea surface salinity (SSS) record between 1989 and 2013 was reconstructed. • SSS in the southern South China Sea is dominated by vertical mixing and freshwater flux. [ABSTRACT FROM AUTHOR]

Tridacna spp. usually have annual and even daily growth bands, and thus provide great potential to be the high-resolution recorders of past climate and environmental changes of the ocean. However, current studies mainly focus on the geochemical records of Tridacna shells, the environmental and climatic implications of long time sequenced daily growth increments are relatively unexplored. In this study, based on the confocal microscopic line-scanning imaging, a 23.7-year long (from 5 June 1989 to 21 February 2013) daily growth rate (DGR) record of Tridacna gigas shell MD3 from southern South China Sea was established and its climatic implication was investigated. On seasonal scale, significant positive correlation was found between the DGR and sea surface temperature (SST). The main growing season of T. gigas MD3 was from March to August, when sufficient sunlight and warmer SST prevailed. While high rainfall (low sunlight) from September to December and low SST from December to March hindered the growth of the shell in autumn and winter. On an inter-annual scale, the DGR of the shell varied periodically on timescales of 2–3 years and 4–5 years, which possibly represented the impacts of the Tropospheric Biennial Oscillation and El Niño-Southern Oscillation activities respectively. Our result suggested that the daily growth lines of T. gigas can establish an extraordinarily high-resolution chronology and have high potential to explore daily to inter-annual paleoenvironmental changes in tropical ocean. Compared with the paleoclimate reconstructions of geochemical approaches, this method has the advantages of low cost and short experimental time. • A modern Tridacna shell from southern SCS has clear daily growth bands. • 23.7-year long daily growth rate (DGR) profile was obtained. • Seasonal variation of the DGR was positively correlated with local SST. • DGR has potential to reconstruct daily to inter-annual paleoclimate changes. • DGR analysis is simpler, faster and less cost compared to geochemical approaches. [ABSTRACT FROM AUTHOR]

Abstract: Intense tectonic movement occurred along the northeastern margin of the Tibetan Plateau 3–4Ma. During the same time period, global climate changed from a period of stability to a period of frequent and abrupt changes, which prevented fluvial and glacial systems from establishing a state of equilibrium. Thus, it is difficult to directly attribute the 3–4Ma coarse-grained sediment accumulation in East Asia to climate change or tectonic activity. This study compares Late Cenozoic multi-proxy records from the Chinese Loess Plateau (CLP) to the low-latitude grain size, sea level and benthic δ 18O records from the South China Sea (SCS) and the high latitude grain-size record from Lake Baikal. All records suggest a steady and persistent cooling since ~4Ma, which correlates well with those cooling records from other regions around the world. This coincided in timing with the modeling results that the closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation between 4.8 and 4.0Ma which led to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH). Cooling of the SH would induce a marked development of the Antarctic ice sheets at ~4Ma, pushing the Intertropical Convergence Zone (ITCZ) northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained sediments in the Sikouzi section from the western CLP since 4.2Ma. Notably, the absence of coarse-grained sedimentation in the Sikouzi section during 3.0-2.1Ma and the gradual decrease in coarse-grained peaks since 2.1Ma reflected by the Sikouzi grain-size record probably resulted from stepwise increase in the NH ice volume pushing the ITCZ southward. On the other hand, development of the Antarctic ice sheets would induce global cooling and enhancement of physical weathering, initiating increases in sedimentation rates as well as increases in grain size from Lake Baikal to the CLP to the SCS. Therefore the closure of the CAS during 4.8–4.0Ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4Ma. A persistent and steady cooling during 4–3Ma probably made a significant contribution to the establishment of the NH ice sheets at 2.75Ma ago. [Copyright &y& Elsevier]