Browsing by Author "Potts, DC"

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    Deglacial mesophotic reef demise on the Great Barrier Reef
    (Elsevier Science BV, 2013-12-15) Abbey, E; Webster, JM; Braga, JC; Jacobsen, GE; Thorogood, GJ; Thomas, AL; Camoin, G; Reimer, PJ; Potts, DC
    Submerged reefs are important recorders of palaeo-environments and sea-level change, and provide a substrate for modem mesophotic (deep-water, light-dependent) coral communities. Mesophotic reefs are rarely, if ever, described from the fossil record and nothing is known of their long-term record on Great Barrier Reef (GBR). Sedimentological and palaeo-ecological analyses coupled with 67 C-14 AMS and U-Th radiometric dates from dredged coral, algae and btyozoan specimens, recovered from depths of 45 to 130 m, reveal two distinct generations of fossil mesophotic coral community development on the submerged shelf edge reefs of the GBR. They occurred from 13 to 10 ka and 8 ka to present. We identified eleven sedimentary fades representing both autochthonous (in situ) and allochthonous (detrital) genesis, and their palaeo-environmental settings have been interpreted based on their sedimentological characteristics, biological assemblages, and the distribution of similar modern biota within the dredges. Facies on the shelf edge represent deep sedimentary environments, primarily forereef slope and open platform settings in palaeo-water depths of 45-95 m. Two coral-algal assemblages and one non-coral encruster assemblage were identified: 1) Massive and tabular corals including Porites, Montipora and faviids associated with Lithophylloids and minor Mastophoroids, 2) platy and encrusting corals including Porites, Montipora and Pachyseris associated with melobesioids and Sporolithon, and 3) Melobesiods and Sporolithon with acervulinids (foraminifera) and bryozoans. Based on their modem occurrence on the GBR and Coral Sea and modem specimens collected in dredges, these are interpreted as representing palaeo-water depths of <60 m, <80-100 m and >100 m respectively. The first mesophotic generation developed at modern depths of 85-130 m from 13 to 10.2 ka and exhibit a deepening succession of <60 to >100 m palaeo-water depth through time. The second generation developed at depths of 45-70 m on the shelf edge from 7.8 ka to present and exhibit stable environmental conditions through time. The apparent hiatus that interrupted the mesophotic coral communities coincided with the timing of modem reef initiation on the GBR as well as a wide-spread flux of siliciclastic sediments from the shelf to the basin. For the first time we have observed the response of mesophotic reef communities to millennial scale environmental perturbations, within the context of global sea-level rise and environmental changes. © 2013, Elsevier Ltd.
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    IODP Expedition 325 to the Great Barrier Reef: unlocking the history of reef growth and demise since the Last Glacial Maximum
    (Australian Geosciences Council, 2012-08-05) Webster, JM; Braga, JC; Humblet, M; Potts, DC; Iryu, Y; Hinestrosa, G; Bourillot, R; Seard, C; Camoin, G; Yokayama, Y; Thomas, AL; Thompson, B; Esat, TM; Fallon, SJ; Dutton, A
    Predicting how the Great Barrier Reef (GBR) will respond to future global climate changes and over what time frame is crucial. Fossil reefs record critical data on geomorphic and ecological consequences of both long-term and abrupt centennial-millennial scale environmental changes. The Integrated Ocean Drilling Program (IODP) Exp. 325 investigated a succession of submerged fossil reefs on the shelf edge of the GBR to establish the course of sea-level change, define sea-surface temperature variations but also analyse the impact of these environmental changes on reef growth since the Last Glacial Maximum (LGM). Thirty-four boreholes were cored from 17 sites along four transects at three locations (Hydrographers Passage, Noggin Pass and Ribbon Reef) in water depths between 42 to 167 m. These cores record responses of the GBR to past environmental stresses similar to current scenarios of future climate change (i.e. changing sea-levels, SST’s, water quality). Initial lithologic, biologic and chronologic data document an active coral reef system that grew, drowned and backstepped up-slope as sea level rose since the LGM. We present an overview of the main Exp. 325 results, including a synthesis of the dating, paleoclimate and reef response team’s findings, in the context of the available site survey data (bathymetry, seismic, seabed imagery). Finally, we discuss the broader implications of these data for understanding how the geometry, composition and development of the GBR responded to repeated and major environmental disturbances since the LGM.
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    Morphology and evolution of drowned carbonate terraces during the last two interglacial cycles, off Hilo, NE Hawaii
    (Elsevier, 2016-01-01) Puga-Bernabéu, Á; Webster, JM; Braga, JC; Clague, DA; Dutton, A; Eggins, SM; Fallon, SJ; Jacobsen, GE; Paduan, JB; Potts, DC
    The eastern side of Hawaii Island is a rapidly subsiding margin dominated by drowned carbonate platforms. We present detailed bathymetric and backscatter data, remotely operated vehicle and submersible observations, sedimentological and 14C accelerator mass spectrometry and U/Th age data from seven submerged terraces (H7, H2a–d, H1a–b) in water depths between 1100 and 25 m off Hilo, north-eastern Hawaii. The main carbonate deposits on these terraces are coral deposits, rhodolith beds, coralline algal mounds, crusts, pavements and tabular sheets. We identified five previously described sedimentary shallow- to deep-water facies and one new facies type that are consistent with reef drowning on a rapidly subsiding margin. We used palaeobathymetric data derived from the sedimentary facies, age versus depth relationships, and published sea-level curves, to estimate a uniform long-term subsidence rate of 2.80 ± 0.36 m/ky for the eastern side of Hawaii over the last 150 ky. Terrace H7 developed about 380 ka based on data from the western side of the island. Active coral growth on terrace H2d occurred during the Marine Isotope Stage (MIS) 6 to 5 transition, and the terrace drowned during the peak of MIS 5e when sea level rose faster than reefs could grow. Favoured by the gentle platform gradient, reefs established progressively landwards with a backstepping pattern during MIS 5e to form the terraces H2c and H2b 122 ka. Final turn-off of shallow water carbonate production on terraces H2b–d coincided with the relative sea-level rise of the interstadial MIS 5a. Bathymetry and submersible data suggest that carbonate sediments on terraces H2a and H1b were deposited over an antecedent topography of local lava deltas emplaced during rising sea levels at ca. 85 and 65 ka, while terrace H1a established on lava delta substrates of the Mauna Loa volcano ca. 11 ka. We conclude that the initiation, growth and drowning of coral-reef terraces off Hilo differ in some ways from the pattern observed in the submerged terraces in the western side of Hawaii and that the platform evolution off Hilo is more strongly influenced by emplacement of offshore lava flows. © 2015, Elsevier B.V