El Ni?o/Southern Oscillation during the 4.2 ka event recorded by growth rates of corals from the North South China Sea

The 4.2 ka event that occurred during the period from 4 500–3 900 a BP was characterized by cold and dry climates and resulted in the collapse of civilizations around the world. The cause of this climatic event, however, has been under debate. We collected four corals(Porites lutea) from Yongxing Island, Xisha Islands, South China Sea, dated them with the U-series method, and measured the annual coral growth rates using X-ray technology. The dating results showed that the coral growth ages were from 4 500–3 900 a BP, which coincide well with the period of the4.2 ka event. We then reconstructed annual sea surface temperature anomaly(SSTA) variations based on the coral growth rates. The growth rate-based SSTA results showed that the interdecadal SSTA from 4 500–3 900 a BP was lower than that during modern times(1961–2008 AD). A spectral analysis showed that the SSTA variations from4 500–3 900 a BP were under the influence of El Nino-Southern Oscillation(ENSO) activities. From 4 500–4 100 a BP, the climate exhibited La Nina-like conditions with weak ENSO intensity and relatively stable and lower SSTA amplitudes. From 4 100–3 900 a BP, the climate underwent a complicated period of ENSO variability and showed alternating El Nino-or La Nina-like conditions at interdecadal time scales and large SSTA amplitudes. We speculate that during the early and middle stages of the 4.2 ka event, the cold climate caused by weak ENSO activities largely weakened social productivity. Then, during the end stages of the 4.2 ka event, the repeated fluctuations in the ENSO intensity caused frequent extreme weather events, resulting in the collapse of civilizations worldwide. Thus, the new evidence obtained from our coral records suggests that the 4.2 ka event as well as the related collapse of civilizations were very likely driven by ENSO variability.

Comparison of Two Reef Sites on the North Coast of Jamaica over a 15-Year Period

This paper compares two reef sites near Discovery Bay, Jamaica, Dairy Bull and Dancing Lady, from 2000 to 2015. At Dairy Bull reef, with low macroalgal cover (8% in 2002 falling to 1% in 2015) and significant number of Diadema antillarum urchins (c. 5 m-2), live coral cover increased from 13% ± 5% in 2006 after the bleaching event in 2005, to 31% ± 7% in 2008, while live Acropora cervicornis increased from 2% ± 2% in 2006 to 28% ± 5% in 2015. Coral cover levels were at least maintained until 2015, owing mostly to a slight increase in A. cervicornis. Dancing Lady reef however was dominated by macroalgae throughout this period (cover of c. 76% ± 7%), with no D. antillarum and showed little decrease in the already low (6% ± 1%) coral cover in 2005. Growth rates for Siderastrea siderea were similar for both sites (7 mm·yr-1), while growth rates of A. cervicornis and A. palmata were 120.0 ± 30 mm·yr-1 and 71.0 ± 29 mm·yr-1 respectively at Dairy Bull in 2015. At Dancing Lady reef, A. cervicornis colonies which were present from 2003 to 2005 had disappeared in 2006, possibly as a result of the mass bleaching event. It appears that A. cervicornis was the most impacted species during the 2005 bleaching event, but was also the species that recovered fastest after its decline at Dairy Bull.