Browsing by Author "Lough, JM"

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    Development of an inshore fringing coral reef using textural, compositional and stratigraphic data from Magnetic Island, Great Barrier Reef, Australia
    (Elsevier, 2012-03-01) Lewis, SE; Wüst, RAJ; Webster, JM; Shields, GA; Renema, W; Lough, JM; Jacobsen, GE
    The stratigraphy of fringing coral reef environments and platforms provides unique insights into reef development and evolution. This study used twelve sediment cores from three transects across a reef flat in Nelly Bay, Magnetic Island (NE Australia), to examine its development over the Holocene. The maximum thickness of the Holocene reefal material based on seismic and bore log data was around 5 m and comparable to other fringing reefs in the Great Barrier Reef (GBR). Six different sedimentary facies were identified in the cores and two coincided with reef accretion: the lower muddy sand with coral rubble and the upper siliciclastic sand with coral rubble. Radiocarbon and U-series dates show that the Nelly Bay fringing reef initiated around 6300 yr BP on a gently sloping, unconsolidated or weakly cemented Pleistocene alluvial sedimentary facies. The ages of four massive coral heads (range between 5790 and 6290 calibrated C-14 yr BP) were almost contemporaneous with reefal initiation indicating that conditions were favourable for reef growth around this time. Vertical accretion rates for the Nelly Bay fringing reef were, on average about 0.5 mm/yr over the last 6000 yr, although rates were as high as 5 mm/yr during initiation. Average lateral accretion rates varied from 98 to 120 mm/yr, which are comparable to rates of other fringing reefs in the region. The age structure of the lower muddy sedimentary facies was consistent with the classic seaward prograding model of fringing reef development. In contrast, the upper siliciclastic sand with coral rubble facies was much younger in age (< 1100 yr BP) than the models and appears to represent an erosional unconformity between the upper and lower reefal facies possibly linked to late Holocene sea-level fall. This younger facies extends laterally across the reef flat and has no obvious prograding symmetry. Our integrated approach, using multi-proxy analyses of sediment cores from the Nelly Bay reef flat, highlights the complex development of this reef which include changes in coral composition and rubble preservation, reef evolution in response to sea-level change and changes in reef accretion rates. (C) 2012 Elsevier Ltd.
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    Has nitrogen supply to coral reefs in the south Pacific Ocean changed over the past 50 thousand years?
    (John Wiley & Sons, Inc, 2019-03-28) Erler, DV; Shepherd, BO; Linsley, BK; Nothdurft, LD; Hua, Q; Lough, JM
    Tropical islands can facilitate surface ocean biological productivity by enhancing the supply of nitrogen to the euphotic zone. Yet in the world's most oligotrophic oceanic region, the South Pacific Subtropical Gyre (SPSG), this “island mass effect” appears diminished. If this is the case, where did island coral reefs in the SPSG get their nitrogen from, and has this changed over time? Here we use coral skeleton isotopes (δ15N and δ18O) and element/Ca ratios to identify the sources of nitrogen to a late Pleistocene coral reef in the SPSG (Cook Islands); we then compare these results to modern corals in the same region. The supply of nitrogen to the late Pleistocene reef appears dominated by upwelling of subsurface nitrogen during cool dry events, supplemented with nitrogen from island-induced N2 fixation (27 ± 3%) during warm wet periods. For the modern corals, N2 fixation supplies nitrogen to the island reefs during cool dry periods with groundwater providing nitrogen during wet periods. We propose that the subsurface supply of nitrogen to the modern reefs has declined as a result of reduced upwelling but this supply has been replaced with increasing nitrogen discharge from groundwater and an increase in island-induced N2 fixation. ©2019 American Geophysical Union