Abstract

The Chiba composite section (CbCS), in the middle of the Boso Peninsula in central Japan, was ratified as the Global Boundary Stratotype Section and Point (GSSP) for the Lower–Middle Pleistocene boundary, accompanied by the Matuyama–Brunhes (M–B) paleomagnetic polarity boundary in January 2020. This study examined the calcareous nannofossil biostratigraphy of the CbCS to describe potential nannofossil events and discuss sea-surface environments around the M–B paleomagnetic polarity boundary. There are no clear biohorizons at the M–B paleomagnetic polarity boundary, although a temporary disappearance of Gephyrocapsa specimens (≥ 5 μm in diameter), an important calcareous nannofossil genus in the Pleistocene, occurs just above the Lower–Middle Pleistocene boundary. Although this is a characteristic event around the M–B paleomagnetic polarity boundary, it is unclear whether the event is globally traceable.Changes in the environmental proxy taxa of calcareous nannofossils in the CbCS revealed that sea-surface environments were driven by glacial-interglacial and millennial-scale climate forces. The time-transgressive change of the Tn value, a calcareous nannofossil temperature index, is mostly concordant with the planktonic foraminiferal oxygen isotope fluctuation. Abundant occurrences of a warm-water species, Umbilicosphaera spp., indicate that the Kuroshio Current was strong after ~ 783 ka. Even the strong influence of the Kuroshio Current, cooling events related to southward movements of the Kuroshio Front occurred every 10,000 years based on the presence of a cold-water taxon, Coccolithus pelagicus. Additionally, the inflow of coastal waters strengthened after ~ 778 ka because of abundant occurrences of Helicosphaera spp. Millennial-scale sea-surface changes were also inferred from detected floral fluctuations of less than 10,000 years.

Graphical abstract

中文翻译：

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GSSP下中新世界的钙质纳米化石生物地层，日本中部和草组国本组千叶组合断面的千叶组合剖面及其对海表环境变化的影响

摘要

位于日本中部Boso半岛中部的千叶组合剖面（CbCS）被批准为下中新世边界的全球边界层标剖面和点（GSSP），并伴有Matuyama-Brunhes（M–B ）在2020年1月的古磁极性边界。这项研究检查了CbCS的钙质纳米化石生物地层，以描述潜在的纳米化石事件，并讨论了M–B古磁极性边界附近的海面环境。尽管Gephyrocapsa暂时消失，但在M–B古磁极性边界没有明确的生物水平。标本（直径≥5μm）是更新世中重要的钙质纳米化石属，出现在下中更新世边界的上方。尽管这是一个围绕M–B古地磁极性边界的特征性事件，但尚不清楚该事件是否可全球追踪。CbCS中钙质纳米化石的环境代用类群的变化表明，海面环境是由冰河间冰期和千禧年规模的气候力量。Tn值（钙质纳米化石温度指数）的随时间变化主要与浮游有孔虫的氧同位素波动一致。大量出现的温水物种伞形目spp。，表明在〜783 ka之后，黑潮潮流很强。即使是黑潮潮流的强大影响，与黑潮阵线南移有关的降温事件也每10,000年发生一次，这是由于存在冷水类群Coccolithus pelagicus而引起的。此外，由于Helicosphaera spp的大量出现，沿海水域的流入量在〜778 ka之后有所增加。还从检测到的小于10,000年的花卉波动中推断出千禧年尺度的海面变化。

图形概要