Climatic and environmental reconstruction based on Holocene coral reefs on the Central Vietnamese coast, western South China Sea

• Main Author: Ai Nguyen
• Format: Thesis
• Language: English
• Published: The University of Queensland, School of Earth Sciences 2011
• Subjects: coral proxy; coral reefs; East Asian Monsoon; ENSO; Holocene; marine pollution; Nha Trang Bay; Oxygen Isotopes; Rare Earth Elements; Sea-level; South China Sea; Strontium/Calcium; Viet Nam; U-series Dating; 0402 Geochemistry; 0403 Geology; 0406 Physical Geography and Environmental Geoscience; North Atlantic
• Online Access: https://espace.library.uq.edu.au/view/UQ:272346/s4067837_PhD_finalabstract.pdf; https://espace.library.uq.edu.au/view/UQ:272346/s4067837_PhD_finalthesis.pdf; https://espace.library.uq.edu.au/view/UQ:272346

The climate and the environment of our planet are always changing and our knowledge of these changes is still in progress. More studies of past climate and environment should be implemented, especially at the present time when the Earth enters a substantial change, e.g. in temperature, sea-level and environment. These changes are believed to be accelerated by recent anthropogenic activities. A better understanding of past climate and environment will facilitate our planning for appropriate strategies of mitigation against and adaption to ongoing climate and environmental change. In this study, climate changes in sea-level, sea surface temperature (SST) and salinity (SSS) since the mid-Holocene, and recent environmental change were reconstructed using fossil and modern corals collected from the Central Vietnamese coast, western South China Sea (SCS). Information on sea-level change on the Vietnamese coast since the mid-Holocene is problematic due to a lack of geochronological data. In this study a high-precision thermal ionisation mass spectrometry (TIMS) U-series technique was used to date the emergent coral reefs on the Central Vietnamese coast. Because coral reefs are considered as a reliable sea-level indicator, the geochronological data are used to constrain the sea-level changes on this coast since the mid-Holocene. The geochronological data suggest three episodes of coral growth for the Central Vietnamese coast: 7395±50 – 5011±16 years BP, 4191±25 – 3122±23 years BP, and 2466±51 – 1695±14 years BP. The first episode accounts for 78% out of 90 dated corals, while each of the two younger episodes accounts for ~10%. The age-elevation data suggest that sea-level was at least +1.74 m at ~7395±50 years BP, reached the maximum highstand of at least +2.5 m ~ 6982 ±40 years BP, then dropped to +1.44 m at ~3122±23 years BP, and to +0.9 m at ~2079 ±29 years BP. The younger corals are distributed seawards, suggesting that the sea-level fell down to the present level after reaching the maximum highstand. The data also show two periods of coral hiatuses (5011±16 – 4191±25 and 3122±23 – 2466±5 years BP), which are intercalated between two episodes of coral growths. Such intercalation suggests oscillating characteristics of the sea-level. The coral hiatuses on the reef flats could be due to the fact that corals during these periods were covered by younger episodes of coral growths, and that during this time the sea-level rose back again or possibly stood still, after falling. The sea-level rise or possibly standstills encouraged the reoccurrence of coral growth on previous substrates that had been exposed to the atmosphere during the falling sea-level. The stability, standstill, or oscillation of the sea-level could be a result of the competing interaction between the uplift of the coast caused by hydro- and geo-isostatic effects following the de-glacial sea-level rise, and younger episodes of sea-level rise contributed by the melt-water discharge in both hemispheres. The second part of this thesis focuses on the use of coral geochemical tracers as climatic proxies for palaeoclimate reconstruction. To accurately reconstruct the climate information recorded in corals, non-climatic factors such as growth rate, terrestrial contamination, coral age (size), and diagenesis, which potentially affect the climatic proxies (Mg/Ca, Sr/Ca, δ13C, and δ18O), were thoroughly assessed. Analysis of correlation with both climatic and non-climatic factors was applied to geochemical data derived from three live corals. The results show that the skeletal extension rate does affect the coral climatic proxies, but it is colony and reef-specific. Chronological correlation suggests terrestrial contamination also contributes to variations of the climatic proxies, and so does the coral age. However, overall it was found that sea surface temperature (SST) plays a dominant role in controlling the variations of climatic proxies, as demonstrated by their significant correlation with instrumental SST and a good agreement of the SST calibrations with those previously published for the north SCS. In addition, the reconstructed SST and seawater δ18O or Δδ18O (calculated by subtracting the thermal components incorporated in coral δ18O) clearly agree with the local and regional climate phenomena, such as the East Asian monsoon (EAM) and El Niño-Southern Oscillation (ENSO). This finding demonstrates a high level of confidence in applying corals from the Central Vietnam coast, western SCS to the study of variations in EAM-ENSO activity and other tropical climate systems. Apart from the non-climatic factors discussed above, coral diagenesis is another major factor that undermines the reliability of climate proxies. Bearing this in mind, diagenesis was carefully inspected for six selected fossil corals with ages ranging from 6987±35 to 1748 ±15 years BP, using petrographical analysis and other geochemical indicators: Mg/Ca, Sr/Ca and δ18O. The results show that post-deposition alteration, such as in-filling of secondary aragonite/calcite cements into the voids, dissolution of skeletal elements, and borings, affects variably most of these corals, but is stronger in the two corals with ages of 5410±34 and 6162±31 years BP. These diagenetic processes would make the constructed SSTs either too low or too high, due to the presence of calcite or aragonite cements in the skeletons. Because of this, the two strongly altered corals, together with the locally altered sections from the other corals, were excluded in climatic interpretation. The least altered massive corals (with ages of 6987±35, 4168±16, 3574±35 and 1748 ±15 years BP) allow reconstruction of the history of EAM activity, and its link to the global climate based on the calibrations obtained from the modern live corals. The results show that the EAM has changed significantly since the mid-Holocene. For instance, the East Asian Summer and Winter Monsoons (EASM/EAWM) were both stronger than the present at ~6987±35 years BP, as shown by larger seasonal amplitudes in SST and seawater δ18O, which is interpreted as being caused by a higher Northern Hemisphere insolation and a reduction/shutdown of North Atlantic meridional overturning circulation (NAMOC). The EAM was dampened at ~4168±16 years BP, and then strengthened at ~3574±35 years BP and at ~1748±15 years BP. The weakened EAM is correlated in time with the Bond event. This suggests a teleconnection between the low- and high-latitude climates. In contrast to the EAM, the ENSO activity at these time windows (6987±35, 4168±16, 3574±35 and 1748 ±15 years BP) appears to behave in the opposite direction, that is, the strengthened EAM correlates with the weakened ENSO. In addition, changes in EAM intensity also resulted in changing rainy seasons. For example, the rainy season at ~4168±16 years BP started 5-6 months earlier than those at ~6987±35 years BP and in the present, while at ~1748±15 years BP the end of the rainy season occurred suddenly. The third part of this thesis focuses on the use of modern massive corals for marine environmental monitoring. In this study, a live Porites coral colony collected from a fringing reef in Nha Trang Bay, Central Vietnam was used to reconstruct local environmental changes over the last 2 decades. Recent coastal development of Nha Trang city has raised public concern over the increasing level of pollution of the bay, and degradation of the coral reefs. This coral covers a time span beyond the onset of the major coastal development in Nha Trang city, enabling us to assess how recent anthropogenic activities have contributed to pollution and degradation of the reef environment. From this coral, a fourteen-year record of trace element data (REEs and other trace elements) at monthly to fortnightly-resolution was obtained back to 1995, showing that the concentrations of trace elements in the corals increased as soon as the coastal development projects commenced. Road, port, and resort construction, port and river dredging, and dumping activities intensified from the end of 2000, and continued until today. Among these activities, the port and river dredging and dumping projects were the most detrimental to Nha Trang Bay. Our results show that when the dredging and dumping occurred, the concentrations of trace elements in the coral dramatically increased, and reached peaks that are comparable to those measured in Porites corals collected in highly polluted places elsewhere in the world. When dumped into the bay, the dredged material persisted in the water column for nearly a year, as demonstrated by the shale-like patterns of the rare earth elements and the significantly reduced Ce anomaly over such periods. The combination of high turbidity, a less oxygenated environment, and possibly more acidic conditions contributed to the deterioration of the reef. A ban on the dumping of dredged material and other wastes should be put in place to save the Nha Trang coral reefs from these human-induced threats.