The Variable Influences of Sea Level, Sedimentation and Exposure on Holocene Reef Development over a Cross-Shelf Transect, Central Great Barrier Reef

Abstract

Coral reefs globally are impacted by natural and anthropogenic stressors that are compounded by climate change. Understanding past reef responses to natural stressors (cyclones, sea-level change, freshwater inputs, and sedimentation) can provide important insights to further understand recent (within the past century) trends in coral cover and diversity. Here we use a compilation of recently published data to investigate the Holocene development of five fringing reefs that are located on a cross-shelf transect on the central Great Barrier Reef, and that are exposed to varying degrees of natural and anthropogenic sedimentation, storm exposure, and Holocene sea-level change. Forty-two reef cores collected using a combination of manual percussion coring and hydraulic drilling techniques, were analysed and dated using uranium-thorium methods. The chronostratigraphic records of reef development established using 105 recently published radiometric ages and seven new uranium-thorium ages from the reef cores and fossil microatolls preserved across the reef flats were compared to investigate cross-shelf variations in reef development. This is the first study to conduct an internal investigation of reef framework across an inshore–offshore gradient to examine the varying levels of influence of sedimentation, sea level and cyclones. Our observations from the central Great Barrier Reef show that reefs furthest offshore from the mainland coast were typically initiated earliest after the post-glacial marine transgression. Reef flat size, morphology, and growth style varied according to constraints placed on reef development by the composition, depth, shape, and relief of the underlying substrate. We establish that terrigenous sedimentation had a marked effect on the development of inshore reefs closest to the mainland (within 10 km of the mainland coast). Periods of relatively high terrigenous sedimentation correspond with enhanced reef accretion rates, and also resulted in a superior record of palaeo-ecological coral composition (i.e., better preservation) at inshore sites. In contrast, mid-Holocene cyclones played a seemingly more important role in the development of reefs >10 km from the mainland; although cyclones clearly affect reefs closer inshore, their geomorphology is affected by a range of controlling factors. Insights provided by these five Holocene reef chronostratigraphies provide useful baseline understanding of reef condition and growth along a cross-shelf transect where the reefs are exposed to variable stressors.

Algal turf sediments on coral reefs: what's known and what's next

“…Furthermore, on geological time scales, there has been significant progress in our understanding of reef growth and development in sediment-rich habitats (e.g. Browne et al, 2013;Perry et al, 2012;Roff et al, 2015;Ryan et al, 2018). This is becoming increasingly topical as sea-level rise becomes an inevitability, along with the associated notion of 'drowned reefs' (Perry et al, 2018;van Woesik et al, 2015).…”

The 4.2 ka event, ENSO, and coral reef development

“…The global impacts of ENSO are primarily a result of the influence of ENSO-based sea-surface temperature anomalies on other drivers of climate variability such as the ITCZ, the Asian monsoon, and the North Atlantic Oscillation (NAO). For example, the warm (cool) tropical Pacific sea-surface temperatures associated with El Niño (La Niña) generally drive a southerly (northerly) shift in the mean position of the ITCZ, which results in changes in precipitation from the tropical Pacific to the neotropics, to Africa (Haug et al, 2001;Chaing et al, 2002;Sachs et al, 2009). Similarly, the Asian monsoon is generally suppressed during El Niño events and enhanced during La Niña events (Liu et al, 2000;Wang et al, 2003).…”

“…There is some evidence that the eastern Pacific mode of ENSO was dominant after ∼ 4.5 ka (Carré et al, 2014), which may explain the muted response to the 4.2 ka event in the central Pacific. Parsing the spatially variable impacts of ENSO and other drivers, such as relative sea level, upwelling, climatic effects other than ENSO, and local physiography and hydrography, to explain patterns of reef development will be a significant challenge going forward (for the Great Barrier Reef: Dechnik et al, 2015;Leonard et al, 2016a, b;Ryan et al, 2018;Webster et al, 2018; see Toth et al, 2016, for the eastern Pacific).…”

Turbid Coral Reefs: Past, Present and Future—A Review

“…A total of 35 published studies have investigated the paleoecological record and/or growth history of turbid reefs with researchers framing questions around three broad themes: (1) the influence of natural drivers, such as regional sea-level oscillations, climate and cyclones on past turbid reef growth and present-day geomorphology [73]; (2) assessing the growth history of reefs in relation to natural and anthropogenic disturbances [72]; and (3) how the growth history and coral community structure of turbid reefs compare to nearby clear-water reefs [74]. Of the 35 published studies, 28 were located in Australia, with 25 of these from the GBR, and the remaining three studies located in the Kimberley and Pilbara regions of Western Australia (Table S1).…”

“…Although reef cores can provide a continuous temporal record of reef growth, the width of the core barrel, typically between 75 and 100 mm in diameter, means that the spatial horizon is vastly underrepresented, and therefore limits a broader paleoecological examination of spatially contemporaneous coral communities through time. Despite this, there are several sedimentary (ratio of terrigenous vs carbonate sediments) [10], paleoecological (clear-water vs turbid-water coral species; coral death assemblages; foraminiferal assemblages) [72,74,80], taphonomic (e.g., style and nature of endolithic borers) [81], and geochemical indicators (e.g., stable isotopes) [82] that when combined with detailed chronostratigraphic analysis can provide information on the local paleo-environments, water quality, climate and coral community structure. For example, increasing suspended sediment load is a key indicator for a change in water quality, typically represented by the relative proportions of carbonate sediments to siliciclastic silts and clays contained within the reef matrix [83].…”