Ho SL et al. (2022), Changing surface ocean circulation caused the l...

ECB-ART-50328

Sci Rep 2022 May 17;121:8204. doi: 10.1038/s41598-022-11920-3.

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Changing surface ocean circulation caused the local demise of echinoid Scaphechinus mirabilis in Taiwan during the Pleistocene-Holocene transition.

Ho SL , Wang JK , Lin YJ , Lin CR , Lee CW , Hsu CH , Chang LY , Wu TH , Tseng CC , Wu HJ , John CM , Oji T , Liu TK , Chen WS , Li P , Fang JN , Lin JP .

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Abundant fossil specimens of Scaphechinus mirabilis, now occurring mostly in temperate waters, have been found in the Toukoshan Formation (Pleistocene) in Miaoli County, Taiwan. Environmental changes leading to its extirpation (local extinction) have thus far been elusive. Here, we reconstruct past environmental and oceanic conditions off northwest Taiwan by analyzing clumped isotopes, as well as stable oxygen isotopes, of well-preserved fossil echinoid tests collected from the Toukoshan Formation. Radiocarbon dates suggest that these samples are from Marine Isotope Stage 3 (MIS 3). Paleotemperature estimates based on clumped isotopes indicate that fossil echinoids were living in oceanic conditions that range from 9 to 14 °C on average, comparable with the estimate derived for a modern sample from Mutsu Bay, Japan. Notably, this temperature range is ~ 10 °C colder than today's conditions off northwest Taiwan. The substantially lower temperatures during ~ 30 ka (MIS 3) compared to the modern conditions might be due to the rerouting of surface currents off northwest Taiwan when the sea level was ~ 60 m lower than today, in addition to the cooling caused by a lower atmospheric CO2 level during the Last Glacial Period. Colder waters brought here by the China Coastal Current (CCC) and the existence of shallow subtidal zones termed "Miaoli Bay" (mainly located in the present-day Miaoli county) during MIS 3 plausibly sustained generations of S. mirabilis, yielding tens of thousands of fossil specimens in the well-preserved fossil beds. The likely extirpation driver is the drastic change from a temperate climate to much warmer conditions in the shallow sea during the Pleistocene-Holocene transition.

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	Figure 1. Fossils and living Scaphechinus mirabilis A. Agassiz, 1864. Scale bars = 10 mm. Aboral views (A,C,E); oral views (B,D,F). (A,B) Fossil specimens (C10), Pleistocene, Miaoli, Taiwan. (C,D) Denuded modern specimen (SM002) used for clumped isotope analyses (Fig. 3B). (E,F) Living specimen (SM100) with intact spines. Figure was created with CorelDRAW X7 Graphic (https://www.coreldraw.com/en/product/coreldraw/).
	Figure 2. Geologic map and legends of stratigraphic units and major structures of studied area are generated based on 1:50,000 geologic maps (Paishatun, Miaoli, Tachia, and Tungshih) released by the Central Geological Survey of Taiwan with additions of fossil localities. Studied specimens are a subset of museum specimens deposited at the Department of Geosciences, National Taiwan University (NTUG) (Ref.5). Map was created with Adobe illustrator CS6 (https://www.adobe.com/products/illustrator.html).
	Figure 3. (A) Violin plots illustrating the distribution of stable oxygen isotopic compositions and (B) temperature estimates derived from clumped isotopes of both fossil and modern specimens of S. mirabilis. The error bars in panel B indicate the uncertainty of the ∆47 temperature for each sample, calculated from the standard error of the measurement (1 sigma, n = 3) and then propagated through the temperature calibration. Due to the non-linearity of ∆47-temperature calibration and the fact that the error estimate is rounded up to the nearest whole number after being converted to temperature unit, the error bars may be asymmetric (e.g. M101A) or one of the error limits may overlap with the mean value (e.g. M101C). See Table S2 for ∆47 data and Methodology section for more detail on the calculation. The red arrows in panel B indicate potential vital effect (0.0146 ± 0.0042‰) as reported by Davies and John10, corresponding to ~ 4.5 °C of underestimation. The gray shaded bar indicates the present-day ocean temperature averaged over the upper 50 m water column off Miaoli based on the climatology data retrieved from the closest grid point in the World Ocean Atlas 2018 (Ref.14)at 24.5° N 120.5° E and the Ocean Data Bank (http://www.odb.ntu.edu.tw/) at 24.5° N 120.75° E (details in Supplementary Information).
	Figure 4. X-ray diffraction analyses of fossil (A) and modern (B) sand dollar S. mirabilis compared with the standard spectrum of high Mg calcite (C). Figure was edited and the line thickness was enhanced with Adobe illustrator CS6 (https://www.adobe.com/products/illustrator.html).
	Figure 5. Surface ocean circulation in the Taiwan Strait at different sea levels. (A) Present-day sea level and (B) when the sea level was lower by ~ 60 m during MIS 3 (30–50 ka) (Ref.16). Color bar in panel A represents ocean bathymetry. In panel B, the area covered by ocean is shaded blue, while land is shaded white. Maps were generated using Ocean Data View (https://odv.awi.de/, version 5.5.1).

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