Kefu Yu

Holocene marine 14C reservoir age variability: Evidence from 230Th-dated corals in the South China Sea

The South China Sea (SCS) is well connected with the western Pacific and influenced by the East Asian monsoon. We have examined temporal variations in radiocarbon marine reservoir ages (R) and regional marine reservoir corrections (Delta R) of the SCS during the Holocene using paired measurements of AMS C-14 and TIMS Th-230 on 20 pristine corals. The results show large fluctuations in both R and Delta R values over the past 7500 years (yrs) with two distinct plateaus during 7.5-5.6 and 3.5-2.5 thousand calendar years before present (cal ka BP). The respective weighted mean Delta R values of these plateaus are 151 +/- 85 and 89 +/- 59 yrs, which are significantly higher than its modern value of -23 +/- 52 yrs. This suggests that using a constant modern Delta R value to calibrate C-14 dates of the SCS marine samples will introduce additional errors to the calibrated ages. Our results provide the first database for the Holocene R and Delta R values of the SCS for improved radiocarbon calibration of marine samples. We interpret the two Delta R plateaus as being related to two intervals with weakened El Nino - Southern Oscillation (ENSO) and intensified East Asian summer monsoon (EASM). This is because the C-14 content of the SCS surface water is controlled by both the C-14 concentration of the Pacific North Equatorial Current (NEC) which is in turn influenced by ENSO-induced upwelling along the Pacific equator and vertical upwelling within the SCS as a result of moisture transportation to midlatitude region to supply the EASM rainfall.

Instability in a marginal coral reef: the shift from natural variability to a human‐dominated seascape

As global climate change drives the demise of tropical reef ecosystems, attention is turning to the suitability of refuge habitat. For the Great Barrier Reef, are there historically stable southern refugia where corals from the north might migrate as climate changes? To address this question, we present a precise chronology of marginal coral reef development from Moreton Bay, southeast Queensland, Australia. Our chronology shows that reef growth was episodic, responding to natural environmental variation throughout the Holocene, and that Moreton Bay was inhospitable to corals for about half of the past 7000 years. The only significant change in coral species composition occurred between ~200 and ~50 years ago, following anthropogenic alterations of the bay and its catchments. Natural historical instability of reefs, coupled with environmental degradation since European colonization, suggests that Moreton Bay offers limited potential as refuge habitat for reef species on human time scales.

Heavy metal pollution recorded in Porites corals from Daya Bay, northern South China Sea.

We examined metal-to-calcium ratios (Fe/Ca, Mn/Ca and Zn/Ca) in the growth bands of two Porites corals from Daya Bay, South China Sea, in order to trace long-term trends in local ambient pollution levels. Although Fe and Mn did not show any obvious increasing trends over 32 years in the period 1976-2007, peak values of Fe/Ca and Mn/Ca occurred in the mid-late 1980s, temporally-coeval with the local construction of a nuclear power station. Furthermore, both corals showed rapid increases in Zn concentrations over the past 14 years (1994-2007), most likely due to increases in domestic and industrial sewage discharge. The Daya Bay corals had higher concentrations of metals than other reported corals from both pristine and seriously polluted locations, suggesting that acute (Fe and Mn) and chronic (Zn) heavy metal contamination has occurred locally over the past approximately 32 years.

Sea surface temperature records in the northern South China Sea from mid‐Holocene coral Sr/Ca ratios

[1] Three mid-Holocene sea surface temperature (SST) records spanning more than 30 years were reconstructed for the northern South China Sea using Sr/Ca ratios in Porites corals. The results indicate warmer than present climates between circa 6100 yr B.P. and circa 6500 yr B.P. with the mid-Holocene average minimum monthly winter SSTs, the average maximum monthly summer SSTs, and the average annual SSTs being about 0.5 degrees-1.4 degrees C, 0 degrees-2.0 degrees C, and 0.2 degrees-1.5 degrees C higher, respectively, than they were during 1970-1994. Summer SSTs decrease from circa 6500 yr B.P. to circa 6100 yr B.P. with a minimum centered at circa 6300 yr B.P. The higher average summer SSTs are consistent with a stronger summer monsoon during the mid-Holocene, and the decreasing trend indicates a secular decrease of summer monsoon strength, which reflects the change in summer insolation in the Northern Hemisphere. El Nino-Southern Oscillation (ENSO) cycles were apparent in both the mid-Holocene coral and modern instrumental records. However, the ENSO variability in the mid-Holocene SSTs was weaker than that in the modern record, and the SST record with the highest summer temperatures from circa 6460 yr B.P. to 6496 yr B.P. shows no robust ENSO cycle. This agrees with other studies that indicate that stronger summer monsoon circulation may have been associated with suppressed ENSO variability during the mid-Holocene.

Coral reefs in the South China Sea: Their response to and records on past environmental changes

This paper reviews both the recent and longer-term (Holocene) ecological history of coral reefs in the South China Sea (SCS). (1) Local ecological monitoring since the 1960s shows that the coral reefs in the South China Sea have declined dramatically, reflecting the rapid decrease of living coral cover and the great loss of symbiotic zooxanthellae. Collectively, this has led to a significant decrease of annual CaCO3 production. Heavy anthropogenic activities and global warming are recognized as major triggers of the observed coral reef degradation. Observations show that the modern coral reefs in the SCS are a source of atmospheric CO2 in summer. (2) Coral reefs of the SCS have been widely used to reveal longer-term environmental variations, including Holocene high-resolution sea surface temperature (SST) and abrupt climate events, millennial-scale El Niño variations, millennial- and centennial-scale sea level oscillations, strong and cyclic storm activities, East Asian monsoon intensities, variation in seawater pH, and recent seawater pollution. (3) Coral reefs of the southern SCS have experienced repeated episodes of bleaching over the last 200 years due to high SST and intense El Niño events; coral reefs of the northern SCS suffered high levels of mortality during several abrupt winter cold-water bleaching events during the middle Holocene warm period. On average, recovery after the middle Holocene cold-bleaching took 20–30 years; recovery following other middle Holocene environmental stresses took approximately 10-20 years. Such findings have significantly contributed to the understanding of the present ecological pressures faced by the coral reefs in the SCS, the histories of Holocene climate/environment changes, and the long-term models of coral reef responses to various past environmental changes.

Paleoprecipitation record from coral Sr/Ca and δ18O during the mid Holocene in the northern South China Sea.

Coupled high-resolution Sr/Ca and δ18O records of a modern and a mid-Holocene coral from Sanya in the southern Hainan Island, northern South China Sea (SCS), were reported and the residual δ18O (Δδ18O) were calculated to indicate precipitation change in this region. Unlike other paleoclimate studies, this study focused on changes of precipitation time rather than precipitation amount. As negative Δδ18O peaks in coral generally correspond to peak precipitations or rainy seasons in the surrounding region, the time offsets between negative Δδ18O peaks and other seasonal indicators, such as sea surface temperature (SST), can well indicate the time of rainy seasons, and the precise time offsets can be estimated by the method of cross spectral analysis. The results of the modern coral indicate that the variation of the coral Δδ18O lags that of the instrumental measured precipitation by about 2 months, and about 3 months to the SST derived from coral Sr/Ca ratios. This agrees well with the modern observation that the salinity change in the southern coastal regions generally lags that of the precipitation in Hianan Island by about 2 months, and the precipitation change lags about 1 month behind the SST in this region. Thus, coral Δδ 18O records can be a reliable proxy for the change of rainy seasons in this region. The results of the mid-Holocene coral show about 2.5 months’ leading of the Δδ18O variation ahead of the SST. By compensating the approximate 3 months’ lag of the Δδ18O variation behind the SST in modern time, the occurrence of rainy seasons during the mid Holocene may have advanced about 5—6 months. In detail, it may start around December and end around April to May with maximum occurring around February. Therefore, rainy seasons mainly occur in winter through early spring during the mid Holocene, compared with that from May through October in modern times. Such precipitation patterns appear to agree with the mid-Holocene pollen records in this region. Variations of large-scale circulation may possibly result in such a different precipitation pattern. Further studies, in particular climate model studies collaborated with meteorologists, are required for a better understanding of the mechanism.

Two centuries-long records of skeletal calcification in massive Porites colonies from Meiji Reef in the southern South China Sea and its responses to atmospheric CO2 and seawater temperature

Rising atmospheric CO2 and warming of the global climate that have occurred since the industrial revolution are regarded as fatal threats to coral reefs. We analyzed the skeletal calcification rate of 14 massive Porites corals from the Meiji Reef in the southern South China Sea through X-ray photography of coral skeletons. A general pattern of change in coral skeletal calcification was determined. The change pattern of coral calcification on the Meiji Reef over the past two centuries can be divided into five periods: calcification increase in 1770–1830, 1870–1920, and 1980–2000 and calcification decline in 1830–1870 and 1920–1980. Over the past two centuries, the largest increase in calcification was 4.5%, occurring in 1770–1830, whereas the largest decline in calcification was 6.2%, occurring in 1920–1980. Coral calcification slightly increased in the recent 20 years (1980–2000). The response relationship of coral calcification to atmospheric CO2 and sea surface temperature (SST) shows that calcification was not correlated with atmospheric CO2 but responded nonlinearly to SST with maxima at ∼27.2°C in 1900–2000. On the Meiji Reef, increasing atmospheric CO2 had a negligible effect on coral growth in the past century. However, rising SST improved coral growth in the early and middle 20th century, and restricted coral growth in the recent 20 years.

Long-term decline of a fringing coral reef in the Northern South China Sea

Abstract Zhao, M.; Yu, K.; Zhang, Q.; Shi, Q., and Price, G.J., 2012. Long-term decline of a fringing coral reef in the northern South China Sea. The Luhuitou coral reef is a fringing reef at Hainan Island in the northern South China Sea. Since the 1960s, the reef has experienced several significant ecological changes. During that interval, the mean coral cover decreased dramatically from 80–90% in 1962–65 and to just 12% in 2009. In the 1960s, the coral community structure was divided into three well-defined zones: a Goniastrea zone and Montipora zone (both on the reef flat) and an Acropora zone (on the reef slope). However, by 2009, Porites lutea became the dominant species on the reef flat, whereas the predominance of Acropora on the reef slope weakened significantly. There are few long-established Porites lutea colonies present, with approximately 80% being younger than 30 years old. This demographic pattern differs significantly from healthy coral reefs, which are typically dominated by large, well-established (and mature) coral colonies. The long-term decline of the Luhuitou coral reef has most likely been driven as a result of anthropogenic activities, such as overfishing, destructive fishing, reef rock digging, and mariculture and tourism activities. Our study reinforces previous works and highlights the vulnerability of coral reefs to anthropogenic impacts.

Timing of Holocene sea-level highstands by mass spectrometric U-series ages of a coral reef from Leizhou Peninsula, South China Sea

Systematic mass spectrometric 230Th ages are reported for aPorites coral reef terrace from Dengloujiao, Leizhou Peninsula, South China Sea. Seven episodes of coral growth were recognized in this terrace: 7125±96, 6764±29, 5826±37, 5006±54, 2543±24,1915±15, and 1513±22 calendar years before present (cal. aBP). 50% of the coral age population fall between 7200 and 6600 cal. aBP, marking post-glacial stabilization of global sea level. Considering the facts that (i) Dengloujiao reef flat was measured at 1.6–2.5 m above modern tidal datum plane; (ii) modernPorites corals in the South China Sea are living at least ∼1 m below the modern tidal datum plane; (iii) the top 20–30 cm of the reef was eroded; and (iv) crustal subsidence in the region since mid-Holocene was negligible, we conclude that the above age groups record at least two major periods (7200–5000 and 2500–1500 cal. aBP) of high sea-level at least 2.9–3.8 m above the present-day level.

U-Series dates of Great Barrier Reef corals suggest at least +0.7 m sea level ~7000 years ago

Strong evidence from field observation and high-precision TIMS U-series dating of exposed in situ massive corals and exposed microatolls on Magnetic Island, Great Barrier Reef, indicates that the relative sea level was at least 0.7 m higher than present ~7000 calendar years (cal yr) ago, and culminated to at least ~1.6 m higher than present ~5800 cal yr ago. The present sea level was probably first reached around 7300—7100 cal yr ago. Together with the youngest age (2236 ± 27 cal yr) of an exposed microatoll colony, the data suggest past sea-level highstands occurred between 7200 and 2200 cal yr ago, lasting for 5000 years. This study represents the first attempt to date sea-level indicators from the Great Barrier Reef by the high-precision U-series method. The results are broadly consistent with a sea-level reconstruction based on a compilation and re-examination of 115 calibrated 14C dates from eastern Australia by Lewis et al. (2008).