Reef-flat and back-reef development in the Great Barrier Reef caused by rapid sea-level fall during the Last Glacial Maximum (30–17 ka)

Abstract

Reef growth patterns and the development of associated environments have been extensively studied from reef deposits from Holocene and previous interglacial highstands. However, reefs that grew during glacial lowstands are comparatively poorly understood. Here we show the formation of reef-flat and back-reef environments following rapid sea-level fall (15–20 mm yr−1 and 20–40 m in magnitude) during the Last Glacial Maximum (LGM) on the present shelf edge of the Great Barrier Reef. Sedimentological and foraminiferal analyses of unconsolidated reef sediments recovered in cores 111–140 m below sea level at Hydrographers Passage during Integrated Ocean Drilling Project (IODP) Expedition 325 reveal the occurrence of a benthic foraminiferal assemblage dominated by the genera Calcarina and Baculogypsina, which is common in modern reef-flat and back-reef environments in the Great Barrier Reef and elsewhere. This assemblage is associated with higher foraminiferal proportions in reef sediments and higher proportions of well-preserved Baculogypsina tests in the same intervals, which also characterize reef-flat environments. Radiocarbon (14C–accelerator mass spectrometry) ages of reef-flat dwelling foraminifers (n = 22), which indicate the time when these foraminifers were alive, are consistent with the timing of the two-step sea-level fall into the LGM as defined by the previously published well-dated coralgal record. This foraminiferal evidence suggests the development of geomorphically mature fringing reefs with shallow back-reef lagoons during the LGM. Our results also imply that back-reef sediment accumulation rates during the LGM lowstand were comparable to those during the Holocene highstand.

Response of large benthic foraminifera to climate and local changes: Implications for future carbonate production

“…Large benthic foraminifera persistence has also been recorded in long‐term sedimentary archives from turbid inshore reefs, prior to and during early European settlement that saw intense changes in land‐use practices in Eastern Australia (Lybolt et al ., 2011; Reymond et al ., 2013b; Narayan et al ., 2015; Johnson et al ., 2017, 2019; Fujita et al ., 2020).…”

“…Detrimental effects of SLR may therefore be mitigated in areas where active reef growth and carbonate production will be maintained and stable, to allow the reef to keep‐up with SLR (Buddemeier & Hopley, 1988; Webb & Kench, 2010; Woodroffe & Webster, 2014). This has been documented in reconstructions of post‐glacial Holocene (transgressive) SLR, resulting in reef initiation, development and persistence of reef communities over time (Reymond et al ., 2011a, 2013b; Fujita et al ., 2020). It is evident that reef ecosystems and low‐light adapted carbonate producers (Renema & Trolestra, 2001; Mateu‐Vicens et al ., 2012; Renema, 2019) can occur under mesophotic conditions and at greater depth ranges than previously thought (Beaman et al ., 1994; Woodroffe & Webster, 2014).…”

“…Carbonate component analysis is a tool for quantifying the relative percentage contribution to carbonate grains by the abundance of multiple, skeleton‐building taxonomic groups in modern and palaeoecological studies (Tudhope & Scoffin, 1988; Lidz & Hallock, 2000; Pomar et al ., 2004; Chazottes et al ., 2008; Ford & Kench, 2012; Reijmer et al ., 2012; Perry et al ., 2014; Reymond et al ., 2014; Morgan & Kench, 2016; Janßsen et al ., 2017). The complementary use of carbonate component analysis has seen an upsurge of research in coral reef settings over the last decade (Renema, 2006b; Narayan & Pandolfi, 2010; Reymond et al ., 2013b; Fajemila et al ., 2015; Narayan et al ., 2015; Pisapia et al ., 2017; Fujita et al ., 2020; Prazeres et al ., 2020a). The LBF and other components derived from surface and sediment core samples, provide invaluable annual, decadal to millennial‐scale records of benthic structure and stability in past reefs (Narayan & Pandolfi, 2011; Reymond et al ., 2013b; Narayan et al ., 2015; Johnson et al ., 2019; Fujita et al ., 2020).…”

“…The LBF contributions to reef carbonate budgets, however, are not strictly reef‐wide estimates. High LBF biomass has been recorded from the high energy reef crest, reef flat and lagoonal environments, both in the modern (Hallock, 1981; Tudhope & Scoffin, 1988; Hohenegger, 1994; Hohenegger et al ., 1999; Fujita et al ., 2009; Doo et al ., 2012, 2017; Mamo, 2016) and in historical settings (Reymond et al ., 2013b; Narayan et al ., 2015; Fujita et al ., 2020). The distribution of common shallow‐water species (i.e.…”

New findings on submerged patch reefs and reefal carbonate rocks at water depths of 70–100 m on the insular shelf off Miyako-jima, southern Ryukyus, Japan

“…Webster et al (2018) reported the reverse of this process, where seaward migration of coral reefs in the Great Barrier Reef was caused by a rapid sea-level fall during the Last Glacial Maximum. In addition, Fujita et al (2019) reported that the development of fringing reefs with shallow back-reef lagoons and the timing of reef-flat and back-reef formation is consistent with the two stepwise falls in sea level during the Last Glacial Maximum. In the SBP profiles of this survey area, the bases of the mound-shaped structures are found shallower than 100 m deep, and strong reflection surfaces that indicate a hard and uneven bottom are recognized continuously with the surfaces of the mound-shaped structures.…”