Yuexing Feng

Palaeoecological evidence of a historical collapse of corals at Pelorus Island, inshore Great Barrier Reef, following European settlement.

The inshore reefs of the Great Barrier Reef (GBR) have undergone significant declines in water quality following European settlement (approx. 1870 AD). However, direct evidence of impacts on coral assemblages is limited by a lack of historical baselines prior to the onset of modern monitoring programmes in the early 1980s. Through palaeoecological reconstructions, we report a previously undocumented historical collapse of Acropora assemblages at Pelorus Island (central GBR). High-precision U-series dating of dead Acropora fragments indicates that this collapse occurred between 1920 and 1955, with few dates obtained after 1980. Prior to this event, our results indicate remarkable long-term stability in coral community structure over centennial scales. We suggest that chronic increases in sediment flux and nutrient loading following European settlement acted as the ultimate cause for the lack of recovery of Acropora assemblages following a series of acute disturbance events (SST anomalies, cyclones and flood events). Evidence for major degradation in reef condition owing to human impacts prior to modern ecological surveys indicates that current monitoring of inshore reefs on the GBR may be predicated on a significantly shifted baseline.

Plume-lithosphere interaction in the generation of the tarim large igneous Province, NW China: Geochronological and geochemical constraints

The magmatism in the early Permian Tarim large igneous province (TLIP) in NW China is represented by basaltic lava flows in Keping and ultramafic-mafic-felsic intrusions and mafic dikes in Bachu, northwestern Tarim Craton. This paper reports new Ar-Ar dating results and chemical compositions of Keping basalts and Bachu dikes, with aims of better characterizing the timing of and mantle/crustal contribution to the TLIP. The Keping basalts yield two well-defined 40Ar/39Ar plateau ages of 287.3 ± 4.0 Ma and 287.9 ± 4.1 Ma, which, together with age data from the literature, define a magmatic event at ∼289 Ma. The intrusions and dikes in Bachu are believed to have formed at ∼279 Ma based on screened literature data. Thus, they together define two magmatic episodes. The Keping basalts, representing the earlier episode, have alkaline affinity (SiO2 = 44.0-47.9 wt.%, Na2O + K2O = 3.7-4.9 wt.%), low MgO (4.3-5.9 wt.%) and high TiO2 (3.8-5.1 wt.%) contents, showing fractionated chondrite-normalized LREE and nearly flat HREE patterns [(La/Yb)N = 6.27-7.71; (Dy/Yb)N = 1.36-1.48] with noticeable negative Nb and Ta anomalies in the primitive mantle-normalized trace element diagram. They have negative and relatively uniform εNd(t) (−2.3 to −3.8) and low (206Pb/204Pb)i (17.43-17.57). We argue that these “crustal signatures” cannot be attributed to crustal assimilation because neither εNd(t) nor (206Pb/204Pb)i correlates with SiO2; rather they are more likely derived from a sub-continental lithospheric mantle (SCLM) source metasomatized by subduction-related processes. The Bachu dikes, representing the later episode and confined to the margins of the Tarim Craton, have similar MgO (3.6-5.4 wt.%) and TiO2 (3.1-4.7 wt.%) contents to the Keping basalts, and display more fractionated REE patterns [(La/Yb)N = 10.1-14.0; (Dy/Yb)N = 1.79-1.99]. They have variable isotope compositions [εNd(t) = −0.3-4.8, (206Pb/204Pb)i = 17.50-18.11] and display OIB-like trace element signatures. Correlations between isotopic and trace element ratios indicate that some dikes with low εNd(t) and low initial Pb isotope ratios could have been subjected to crustal assimilation. We propose a model involving plume-lithosphere interaction to account for the two discrete magmatic episodes with distinct mantle sources in the TLIP. The earlier episode was formed in response to the impact of a sub-lithospheric mantle plume at the base of the SCLM. Partial melting of the metasomatized lithospheric mantle was triggered by temperature increase due to conductive heating of the impregnating mantle plume. The later episode was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim Craton with thinner lithosphere.

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.

Past 140-year environmental record in the northern South China Sea: Evidence from coral skeletal trace metal variations

About 140-year changes in the trace metals in Porites coral samples from two locations in the northern South China Sea were investigated. Results of PCA analyses suggest that near the coast, terrestrial input impacted behavior of trace metals by 28.4%, impact of Sea Surface Temperature (SST) was 19.0%, contribution of war and infrastructure were 14.4% and 15.6% respectively. But for a location in the open sea, contribution of War and SST reached 33.2% and 16.5%, while activities of infrastructure and guano exploration reached 13.2% and 14.7%. While the spatiotemporal change model of Cu, Cd and Pb in seawater of the north area of South China Sea during 1986-1997 were reconstructed. It was found that in the sea area Cu and Cd contaminations were distributed near the coast while areas around Sanya, Hainan had high Pb levels because of the well-developed tourism related activities.

Quaternary continental weathering geochronology by laser-heating 40Ar/39Ar analysis of supergene cryptomelane

Incremental laser-heating analyses of supergene cryptomelane clusters extracted from three distinct weathering profiles from the Mary Valley region, southeast Queensland, Australia, yield reproducible and well-defined plateau ages ranging from 346 +/- 15 to 291 +/- 14 ka (2 a). Precipitation of supergene cryptomelane in this period implies that relative humid climate prevailed in southeast Queensland from 340 to 290 ha, a result consistent with oxygen isotope analyses of marine sediments from Ocean Drilling Program Site 820 and with regional pollen and spore records. These results, the first report on the precise Ar-40/Ar-39 dating of Quaternary supergene cryptomelane, indicate that Ar-40/Ar-39 analysis of pedogenic minerals provides a reliable geochronometer for the study of Quaternary surficial processes useful in the study of soil formation rates, continental paleoclimates, and archaeological sites devoid of datable volcanic minerals.

Mid-Holocene sea-level and coral reef demise: U-Th dating of subfossil corals in Moreton Bay, Australia:

It is increasingly apparent that sea-level data (e.g. microfossil transfer functions, dated coral microatolls and direct observations from satellite and tidal gauges) vary temporally and spatially at regional to local scales, thus limiting our ability to model future sea-level rise for many regions. Understanding sea-level response at ‘far-field’ locations at regional scales is fundamental for formulating more relevant sea-level rise susceptibility models within these regions under future global change projections. Fossil corals and reefs in particular are valuable tools for reconstructing past sea levels and possible environmental phase shifts beyond the temporal constraints of instrumental records. This study used abundant surface geochronological data based on in situ subfossil corals and precise elevation surveys to determine previous sea level in Moreton Bay, eastern Australia, a far-field site. A total of 64 U-Th dates show that relative sea level was at least 1.1 m above modern lowest astronomical tide (LAT) from at least ~6600 cal. yr BP. Furthermore, a rapid synchronous demise in coral reef growth occurred in Moreton Bay ~5800 cal. yr BP, coinciding with reported reef hiatus periods in other areas around the Indo-Pacific region. Evaluating past reef growth patterns and phases allows for a better interpretation of anthropogenic forcing versus natural environmental/climatic cycles that effect reef formation and demise at all scales and may allow better prediction of reef response to future global change.

U-Th dating of striated fault planes

Direct dating of brittle fault activity is of fundamental importance to tectonic reconstructions and paleoseismic studies. One way to address this issue is by constraining the timing of fault striations, but this requires a better understanding of the striation formation mechanism and associated mineralization. We present results from a microstructural, geochemical, and geochronological study of calcite precipitates associated with striated fault planes from the Dead Sea fault zone in northern Israel. We recognize four types of coexisting calcite precipitates, including calcite cement in dilation breccia, calcite in striated groove morphology, calcite gouge associated with hydraulic fracturing and pressure solution, and calcite coating of the fault surface. Carbon-oxygen isotopes, 87Sr/86Sr ratios, and rare earth element and yttrium (REY) patterns indicate various precipitation mechanisms associated with formation of syntectonic (calcite cement and striations), coseismic (calcite gouge), and interseismic (calcite coating) precipitates in the fault zone. Using U-Th dating of samples from three adjacent fault planes, we delineate four welldefined deformation ages in the period from 220 to 60 ka. We conclude that these ages constrain the timing of activity along the Dead Sea fault zone in northern Israel, and argue that a similar methodological approach could potentially shed light on the timing of deformation in other brittle fault zones.

An 80 kyr-long continuous speleothem record from Dim Cave, SW Turkey with paleoclimatic implications for the Eastern Mediterranean

Speleothem-based stable isotope records are valuable in sub-humid and semi-arid settings where many other terrestrial climate proxies are fragmentary. The Eastern Mediterranean is one such region. Here we present an 80-kyr-long precisely-dated (by U-series) and high-resolution oxygen (δ18O) and carbon (δ13C) records from Dim Cave (~36°N) in SW Turkey. The glacial-interglacial δ18O variations in the Dim Cave speleothem are best explained in terms of changes in the trajectories of winter westerly air masses. These are along a northerly (European) track (isotopically less depleted) during the early last glaciation but are gradually depressed southward closer to the modern westerly track along the North African coast (more depleted) after c.50 kyr and remain in the southern track through the Last Glacial Maximum. The southward displacement of the westerly track reflects growth of the Fennoscandian ice sheet and its impact on westerly wind fields. Changes in δ13C are interpreted as reflecting soil organic matter composition and/or thickness. δ13C values are significantly more negative in interglacials reflecting active carbonic acid production in the soil and less negative in glacial times reflecting carbonate rock values. Several Heinrich events are recorded in the Dim record indicating intensification of westerly flow across this part of the EM.

Faces of the ancestors revealed: Discovery and dating of a Pleistocene-age petroglyph in Lene Hara Cave, East Timor

A petroglyph showing a human face found in East Timor is dated to the late Pleistocene. It recalls ancient Australian forms and raises the possibility of connecting early cave art with the better known painted figures of Lapita/Austronesian art ten millennia later. This new discovery at a known cave shows what precious evidence still lies in store even in well-trodden places.

Cook Island artifact geochemistry demonstrates spatial and temporal extent of pre-European interarchipelago voyaging in East Polynesia

Significance Oceania, the last region settled on Earth, witnessed the greatest maritime migration in human history. Scholars have debated how and when islands were colonized and the role of postsettlement voyaging in maintaining founding colonies and in subsequent diversification of island societies. We geochemically “fingerprinted” exotic stone artifacts from a well-dated archaeological site in the Cook Islands, matching artifacts to their geological sources and demonstrating that the geographical voyaging network extended beyond the Cook Islands to include the Austral, Samoa, and Marquesas archipelagos—up to 2,400 km distant. We further demonstrate that Polynesian interarchipelago voyaging lasted from about AD 1300 to the 1600s, suggesting that long-distance interaction continued to influence the development of social structures in East Polynesia well after initial colonization. The Cook Islands are considered the “gateway” for human colonization of East Polynesia, the final chapter of Oceanic settlement and the last major region occupied on Earth. Indeed, East Polynesia witnessed the culmination of the greatest maritime migration in human history. Perennial debates have critiqued whether Oceanic settlement was purposeful or accidental, the timing and pathways of colonization, and the nature and extent of postcolonization voyaging—essential for small founding groups securing a lifeline between parent and daughter communities. Centering on the well-dated Tangatatau rockshelter, Mangaia, Southern Cook Islands, we charted the temporal duration and geographic spread of exotic stone adze materials—essential woodworking tools found throughout Polynesia— imported for more than 300 y beginning in the early AD 1300s. Using a technique requiring only 200 mg of sample for the geochemical analysis of trace elements and isotopes of fine-grained basalt adzes, we assigned all artifacts to an island or archipelago of origin. Adze material was identified from the chiefly complex on the Austral Islands, from the major adze quarry complex on Tutuila (Samoa), and from the Marquesas Islands more than 2,400 km distant. This interaction is the only dated example of down-the-line exchange in central East Polynesia where intermediate groups transferred commodities attesting to the interconnectedness and complexity of social relations fostered during postsettlement voyaging. For the Cook Islands, this exchange may have lasted into the 1600s, at least a century later than other East Polynesian archipelagos, suggesting that interarchipelago interaction contributed to the later development of social hierarchies.