Evidence from giant-clam δ 18 O of intense El Ninõ–Southern Oscillation-related variability but reduced frequency 3700 years ago
2020
Abstract. Giant clams (Tridacna) are the largest marine bivalves, and their carbonate shells
can be used for high-resolution paleoclimate reconstructions. In this
contribution, δ18O shell was used to estimate climatic
variation in the Xisha Islands of the South China Sea. We first evaluate sea
surface temperature (SST) and sea surface salinity (SSS) influence on the
modern resampled monthly ( r -monthly) resolution of Tridacna gigas δ18O shell . The results obtained reveal that δ18O shell seasonal variation is mainly controlled by SST and
appears to be insensitive to local SSS change. Thus, the δ18O
of Tridacna shells can be roughly used as a proxy of local SST: a 1 ‰ δ18O shell change is roughly equal to
4.41 ∘ C of SST. The r -monthly δ18O of a 40-year-old
Tridacna squamosa ( 3673±28 BP) from the North Reef of the Xisha Islands was analyzed
and compared with the modern specimen. The difference between the average
δ18O of the fossil Tridacna shell ( δ18O = - 1.34 ‰) and the modern Tridacna specimen ( δ18O = - 1.15 ‰) probably implies a warm climate, roughly
0.84 ∘ C, 3700 years ago. The seasonal variation 3700 years ago
was slightly lower than that suggested by modern instrumental data, and the
transition between warm and cold seasons was rapid. Higher amplitudes of
reconstructed r -monthly and r -annual SST anomalies imply an enhanced climate
variability during this warm period. Investigation of the El
Nino–Southern Oscillation (ENSO) variation (based on the reconstructed
SST series) indicates reduced ENSO frequency but increased ENSO-related
variability and extreme El Nino winter events 3700 years ago.
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