Kevin Welsh

Chronostratigraphic framework for the IODP Expedition 318 cores from the Wilkes Land Margin: constraints for paleoceanographic reconstruction

The Integrated Ocean Drilling Program Expedition 318 to the Wilkes Land margin of Antarctica recovered a sedimentary succession ranging in age from lower Eocene to the Holocene. Excellent stratigraphic control is key to understanding the timing of paleoceanographic events through critical climate intervals. Drill sites recovered the lower and middle Eocene, nearly the entire Oligocene, the Miocene from about 17 Ma, the entire Pliocene and much of the Pleistocene. The paleomagnetic properties are generally suitable for magnetostratigraphic interpretation, with well-behaved demagnetization diagrams, uniform distribution of declinations, and a clear separation into two inclination modes. Although the sequences were discontinuously recovered with many gaps due to coring, and there are hiatuses from sedimentary and tectonic processes, the magnetostratigraphic patterns are in general readily interpretable. Our interpretations are integrated with the diatom, radiolarian, calcareous nannofossils and dinoflagellate cyst (dinocyst) biostratigraphy. The magnetostratigraphy significantly improves the resolution of the chronostratigraphy, particularly in intervals with poor biostratigraphic control. However, Southern Ocean records with reliable magnetostratigraphies are notably scarce, and the data reported here provide an opportunity for improved calibration of the biostratigraphic records. In particular, we provide a rare magnetostratigraphic calibration for dinocyst biostratigraphy in the Paleogene and a substantially improved diatom calibration for the Pliocene. This paper presents the stratigraphic framework for future paleoceanographic proxy records which are being developed for the Wilkes Land margin cores. It further provides tight constraints on the duration of regional hiatuses inferred from seismic surveys of the region.

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.

Eocene-Miocene shallow-water carbonate platforms and increased habitat diversity in Sarawak, Malaysia

ABSTRACT The Indo-Pacific marine biodiversity hotspot originated between the late Eocene and the early Miocene. Its origin coincides with an increase in availability of shallow-marine habitats driven by the opening of the South China Sea and the collision of Australia with the Pacific arcs and the southeast Asian margin. However, little is known about the distribution and diversity of past Indo-Pacific marine habitats. Understanding habitat diversity is key for understanding the significance of biodiversity origins and a necessary prerequisite for interpreting biodiversity patterns through time. Here we describe and interpret past carbonate platform environments in Sarawak, Malaysia during a time of active tectonism. We examine upper Eocene to lower Miocene marine shallow-water carbonate deposits from six localities in two limestone formations: the large ramplike Melinau carbonate platform (middle Eocene to early Miocene) and the unattached Subis carbonate platform (early Miocene). Deposits examined in this study represent paleoenvironments. Our analysis reveals an increase in habitat diversity from the Eocene to the Miocene. Mesophotic to oligophotic low-energy environments are typical for the Eocene sites. The corals first appear in the Oligocene deposits, but genuine reef depositional settings are not observed until the Miocene. This study provides both insight into the evolution of the carbonate platform environments along the Sarawak margin, and context for the origin of the Indo-Pacific marine biodiversity hotspot.

ENSO reconstructions over the past 60 ka using giant clams (Tridacna sp.) from Papua New Guinea

We present 12 seasonally resolved delta O-18 profiles of giant clams (Tridacna sp.) from the Huon Peninsula, Papua New Guinea, spanning discrete periods of time (9-38 years) over the past 60 ka. The interannual anomaly time series of these shells are used to reconstruct interannual variability which can predominantly be attributed in the modern climate to the El Nino-Southern Oscillation (ENSO) in this region. We have found a significant reduction in interannual delta O-18 variance during the early Holocene, whereas during Marine Isotope Stage 3 there were at least some periods with variance not significantly different to the twentieth century. We show that ENSO variability seen during the late twentieth century is rare but not unprecedented within glacial climates.

Evidence of reduced mid-Holocene ENSO variance on the Great Barrier Reef, Australia

Globally, coral reefs are under increasing pressure both through direct anthropogenic influence and increases in climate extremes. Understanding past climate dynamics that negatively affected coral reef growth is imperative for both improving management strategies, and for modelling coral reef responses to a changing climate. The El Nino Southern Oscillation (ENSO) is the primary source of climate variability at inter-annual timescales on the Great Barrier Reef (GBR), north-eastern Australia. Applying continuous wavelet transforms to visually assessed coral luminescence intensity in massive Porites corals from the central GBR we demonstrate that these records reliably reproduce ENSO variance patterns for the period 1880 – 1985. We then applied this method to three sub-fossil corals from the same reef to reconstruct ENSO variance from ~5200 – 4300 years before present (yBP). We show that ENSO events were less extreme and less frequent after ~5200 yBP on the GBR compared to modern records. Growth characteristics of the corals are consistent with cooler sea surface temperatures (SST) between 5200 and 4300 yBP compared to both the millennia prior (~6000 yBP) and modern records. Understanding ENSO dynamics in response to SST variability at geological timescales will be important for improving predictions of future ENSO response to a rapidly warming climate.

Identifying patterns and drivers of coral diversity in the Central Indo-Pacific marine biodiversity hotspot

Abstract. Biodiversity hotspots are increasingly recognized as areas of high taxonomic and functional diversity. These hotspots are dynamic and shift geographically over time in response to environmental change. To identify drivers of the origin, evolution, and persistence of diversity hotspots,we investigated the diversity patterns of reef-building corals (Scleractinia) in the Central Indo-Pacific, a marine biodiversity hotspot for the last 25 Myr. We used the scleractinian fossil record (based on literature and a newly acquired fossil collection) to examine the taxonomic and functional diversity of corals from the Eocene to Pliocene. Our data identify potential drivers of coral diversity through time (and space) in the Central Indo-Pacific by constraining the timing of taxonomic turnover events and correlating them with known environmental changes. Increases in taxonomic diversity, high origination rates, and changes in abundance of functional character states indicate that the origin of the Central Indo-Pacific hotspot occurred during the Oligocene, initially through proliferation of pre-existing taxa and then by origination of new taxa. In contrast to taxonomic diversity, overall functional diversity of Central Indo-Pacific reef-building corals remained constant from the Eocene to the Pliocene. Our results identify global sea level as a main driver of diversity increase that, together with local tectonics, regulates availability of suitable habitats. Moreover, marine biodiversity hotspots develop from both the accumulation of taxa from older biodiversity hotspots and origination of new taxa. Our study demonstrates the utility of a combined literature-based and new collection approach for gaining new insights into the timing, drivers, and development of tropical biodiversity hotspots.

Late-Holocene cliff-top blowout activation and evolution in the Cooloola Sand Mass, south-east Queensland, Australia

Cliff-top dunes are a locally important geomorphic features of sedimentary coasts. They are traditionally interpreted as being sourced by (or with) sand derived from the beach below the cliff. This paper presents the results of a stratigraphic and geochronological study of Carlo Sand Blow, a coastal blowout that has developed on top of a high sandy cliff in the Cooloola Sand Mass, south-east Queensland. We use a combination of sedimentological, pedological and geophysical techniques along with optically stimulated luminescence dating to determine the depositional history and evolution of the blowout. We demonstrate that the blowout is dominantly nourished by sand eroded from its floor rather than the adjacent beach. The original dune surface dates to the first half of the last glacial period (c. 40–70 ka) and this dune was deflated in the late-Holocene. Dune activity is directly associated with cliff undercutting because of coastal retreat in the late-Holocene, but coastal erosion on its own is not capable of maintaining aeolian activity. Blowout activity occurred between 2.6 and 2.3 ka and again at 0.3 ka with aeolian sand burying palaeosols. Both soil surfaces contained charcoal and tree stumps in growth position and our study suggests that fire is the immediate trigger for blowout reactivation. It is likely that these fires were anthropogenic in origin, because the site is somewhat protected from natural fire and the ages coincide with intensification of human use of coastal sites in the area.