Late Oligocene−Miocene evolution of deep-water circulation in the abyssal South China Sea: Insights from Nd isotopes of fossil fish teeth

Geology Pub Date : 2024-05-23 DOI:10.1130/g52042.1
Wei Shu, Christophe Colin, Zhifei Liu, Arnaud Dapoigny
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Abstract

A long-term neodymium isotope (εNd) record of fossil fish teeth was investigated to constrain the evolution of deep-water circulation in the abyssal South China Sea (SCS) during the late Oligocene−Miocene (27−10 Ma). Fish teeth samples were collected from the oceanic red beds at International Ocean Discovery Program Expedition 367 Site U1499 (water depth 3758 m). Seawater εNd values (from −7.1 to −4.8, average −6.1) prior to 15 Ma indicate that water masses in the abyssal SCS resulted from the mixing of more radiogenic Upper Circumpolar Deep Water (UCDW, average −4.5) and less radiogenic Lower Circumpolar Deep Water (LCDW, average −6.4). The general decrease in εNd values was attributed to an increasing influence of the unradiogenic LCDW at the studied site, consistent with the subsidence and the associated deepening of the SCS plain. After 15 Ma, seawater εNd dropped significantly to a range of −8.9 to −6.1 (average −7.5), indicating a slowdown in the hydrological connection between the deep-water masses in the SCS and the western Pacific Ocean. We argue that the formation of the Luzon Strait due to the uplift of the Luzon arc in the late Miocene led to the shallowing and narrowing of the SCS-Pacific channels. Consequently, penetration of LCDW was reduced and water masses in the abyssal SCS would have been less ventilated and strongly influenced by lithogenic input from the unradiogenic sediments of large Asian rivers draining the peri-Himalayan region.
晚渐新世-中新世南海深海环流的演变:从化石鱼齿的钕同位素中获得的启示
研究了化石鱼类牙齿的长期钕同位素(εNd)记录,以确定渐新世-中新世晚期(27-10 Ma)南海深海环流的演化过程。鱼牙样品采集自国际大洋发现计划 367 考察队 U1499 站(水深 3758 米)的大洋红床。15 Ma以前的海水εNd值(从-7.1到-4.8,平均-6.1)表明,深海SCS中的水团是由辐射性较强的上极圈深水(UCDW,平均-4.5)和辐射性较弱的下极圈深水(LCDW,平均-6.4)混合而成的。εNd值的普遍下降是由于在研究地点,非辐射成因的LCDW的影响越来越大,这与南中国海平原的下沉和相关的加深是一致的。15 Ma之后,海水εNd显著下降到-8.9至-6.1(平均-7.5),表明南中国海深水海团与西太平洋之间的水文联系减缓。我们认为,中新世晚期吕宋岛弧的隆起形成了吕宋海峡,导致南中国海-太平洋海峡变浅变窄。因此,低纬度海流的穿透力减弱,深海南中国海的水团通气性降低,并受到排入喜马拉雅山周边地区的亚洲大河的非辐射沉积物的强烈影响。
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