High resolution Secondary Ionisation Mass Spectrometry (SIMS) δ18O analyses of Hulu Cave speleothem at the time of Heinrich Event 1

The suitability of in situ Secondary Ionisation Mass Spectrometry (SIMS) techniques for measuring O isotopes in speleothems is critically examined by applying this technique to a 500-year interval of the well-known Hulu Cave record (Wang, Y.J., Cheng, H, Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.C., Dorale, J.A., 2001. A high-resolution absolute dated late Pleistocene monsoon record from Hulu Cave, China. Science, 294: 2345–2348). This interval includes a large abrupt + 2‰ shift in δ18O at 16.07 ka, which may correlate to Heinrich Event I (H1). The high-resolution SIMS method provides annual to near-annual δ18O data, thereby increasing the temporal resolution of the previously published Hulu Cave isotopic data by approximately tenfold. SIMS δ18O data reveal that 75% of the abrupt isotopic shift at 16.07 ka occurred in just 1 to 2 years and the full + 2‰ occurs over 6 years, compared with an upper limit of 20 years as previously determined by conventional methods employing micro-cutting, acid-digestion and CO2-gas source mass spectrometry methods. SIMS δ18O data also reveal numerous high amplitude (1–3‰), high frequency (< 20 year) fluctuations not resolvable with conventional data that are recorded in the several hundred years prior to 16.07 ka and may persist after the 16.07 ka event. These fluctuations are interpreted to represent more local rainfall changes while the + 2‰ shift represents a rapid change in the underlying δ18O mean driven by more complex processes that are maintained for 500 years.