Late glacial to deglacial variation of coralgal assemblages in the Great Barrier Reef, Australia

Integrated Ocean Drilling Program (IODP) Expedition 325 cored submerged reefs along the shelf edge of the Great Barrier Reef (GBR) to study sea-level and environmental changes and their impacts on reef communities and reef growth since the Last Glacial Maximum (LGM). Previous work defined five reef sequences (Reef 1–5) that span the last 30,000 years. Here we examined the variation in coralgal assemblages and their paleoenvironmental settings in late glacial to deglacial sequences from 23 holes cored seaward of the modern GBR in water depths from 46 to 131 m along four transects at three localities: Hydrographers Passage (HYD-01C and HYD-02A), Noggin Pass (NOG–01B), and Ribbon Reef (RIB-02A). We identified three coralline algal assemblages and eight coral assemblages indicating a broad range of reef settings from the shallow reef crest (0–5 m) to the deep forereef slope (>20 m). We document in detail for the first time the distribution and composition of reef communities that grew in the GBR during the LGM from 22,000–19,000 years ago. They included coral taxa that are major reef builders today: Isopora, Acropora gr. humilis, Dipsastraea gr. pallida, Porites, and Montipora. Prior to the fall in sea level to the maximum extent of the LGM, late glacial reef communities developed more proximally (landward) to the modern GBR along the shelf edge. Their distribution and composition reflect influences of the older Pleistocene basement depth and possible terrigenous sediment inputs. Post-LGM deglacial reef growth was vigorous in proximal sites and characterized by the accretion of a very shallow high-energy coralgal assemblage composed of medium to robustly branching Acropora, including A. gr. humilis, and thick algal crusts of Porolithon gr. onkodes associated with vermetid gastropods. More distally, reef growth was variably impacted by terrigenous input following deglacial reflooding of antecedent reef terraces. The coralgal succession and sedimentary facies in Noggin Pass indicate that an early drowning trend was linked to increased turbidity that was likely controlled by shelf morphology (narrow shelf, steep slope) and/or proximity to a paleo-river mouth. The deglacial succession in Ribbon Reef lacks typical shallow-water indicators, which may reflect influences of the particularly steep slope of the northern GBR shelf edge on reef zonation. A major sea-level jump at the onset of the Younger Dryas displaced reef habitats further upslope, forming a barrier reef system mainly composed of robustly branching acroporids distinct from the more distal sites. Our results highlight the importance of sedimentation and shelf morphology in addition to relative sea-level changes in controlling variations in reef community over centennial to millennial timescales. © 2019 Elsevier B.V.Australian Research Council-DP109400

Response of the Great Barrier Reef to sea level and environmental changes over the past 30,000 years

Previous drilling through submerged fossil coral reefs has greatly improved our understanding of the general pattern of sea-level change since the Last Glacial Maximum, however, how reefs responded to these changes remains uncertain. Here we document the evolution of the Great Barrier Reef (GBR), the world\u27s largest reef system, to major, abrupt environmental changes over the past 30 thousand years based on comprehensive sedimentological, biological and geochronological records from fossil reef cores. We show that reefs migrated seaward as sea level fell to its lowest level during the most recent glaciation (~20.5-20.7 thousand years ago (ka)), then landward as the shelf flooded and ocean temperatures increased during the subsequent deglacial period (~20-10 ka). Growth was interrupted by five reef-death events caused by subaerial exposure or sea-level rise outpacing reef growth. Around 10 ka, the reef drowned as the sea level continued to rise, flooding more of the shelf and causing a higher sediment flux. The GBR\u27s capacity for rapid lateral migration at rates of 0.2-1.5 m yr−1 (and the ability to recruit locally) suggest that, as an ecosystem, the GBR has been more resilient to past sea-level and temperature fluctuations than previously thought, but it has been highly sensitive to increased sediment input over centennial-millennial timescales

Rapid glaciation and a two-step sea-level plunge into The Last Glacial Maximum

The approximately 10,000-year-long Last Glacial Maximum, before the termination of the last ice age, was the coldest period in Earth’s recent climate history1. Relative to the Holocene epoch, atmospheric carbon dioxide was about 100 parts per million lower and tropical sea surface temperatures were about 3 to 5 degrees Celsius lower2,3. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago4 and was followed by about 10,000 years of rapid deglaciation into the Holocene1. The masses of the melting polar ice sheets and the change in ocean volume, and hence in GMSL, are primary constraints for climate models constructed to describe the transition between the Last Glacial Maximum and the Holocene, and future changes; but the rate, timing and magnitude of this transition remain uncertain. Here we show that sea level at the shelf edge of the Great Barrier Reef dropped by around 20 metres between 21,900 and 20,500 years ago, to −118 metres relative to the modern level. Our findings are based on recovered and radiometrically dated fossil corals and coralline algae assemblages, and represent relative sea level at the Great Barrier Reef, rather than GMSL. Subsequently, relative sea level rose at a rate of about 3.5 millimetres per year for around 4,000 years. The rise is consistent with the warming previously observed at 19,000 years ago1,5, but we now show that it occurred just after the 20-metre drop in relative sea level and the related increase in global ice volumes. The detailed structure of our record is robust because the Great Barrier Reef is remote from former ice sheets and tectonic activity. Relative sea level can be influenced by Earth’s response to regional changes in ice and water loadings and may differ greatly from GMSL. Consequently, we used glacio-isostatic models to derive GMSL, and find that the Last Glacial Maximum culminated 20,500 years ago in a GMSL low of about −125 to −130 metres.Financial support of this research was provided by the JSPS KAKENHI (grant numbers JP26247085, JP15KK0151, JP16H06309 and JP17H01168), the Australian Research Council (grant number DP1094001), ANZIC, NERC grant NE/H014136/1 and Institut Polytechnique de Bordeaux

Tahiti sea level : the last deglacial sea level rise in the South Pacific

Integrated Ocean Drilling Program (IODP) Expedition 310 to the reef terraces around Tahiti, French Polynesia, was the second expedition to utilize a mission-specific platform (MSP) and was conducted by the European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO). The objectives of Expedition 310 are to establish the course of postglacial sea level rise at Tahiti, to define sea-surface temperature (SST) variations for the region over the period 20–10 ka, and to analyze the impact of sea level changes on reef growth and geometry. To meet these objectives, the postglacial reef sequence, which consists of successive reef terraces seaward of the living barrier reef, was cored from a dynamically positioned vessel during October and November 2005. A total of 37 boreholes across 22 sites were cored in water depths ranging from 41.65 to 117.54 m. Borehole logging operations in 10 boreholes provided continuous geophysical information about the drilled strata. The cores were described during the Onshore Science Party at the IODP Bremen Core Repository during February and March 2006, where minimum and some standard measurements were made. Further postcruise research on samples taken during the Onshore Science Party are expected to fulfill the objectives of the expedition

Tahiti sea level expedition : the last deglacial sea level rise in the South Pacific

The history of sea level and sea-surface temperature (SST) variation associated with the last deglaciation is of prime interest to understanding dynamics of large ice sheets and their effects on Earth’s isostasy. So far, the only sea level record that encompasses the whole deglaciation is based on offshore drilling of Barbados coral reefs that overlie an active subduction zone, implying that the apparent sea level record may be biased by tectonic movements. This proposal seeks to establish the course and effects of the last deglaciation in a reef setting that developed in a tectonically inactive area at sites located far away from glaciated regions in Tahiti (French Polynesia). At each site, we propose to sample a transect of several offshore drill holes using a dynamically positioned drilling vessel. The study will have three major objectives. The first objective will be to reconstruct the deglaciation curve for the period 20.00–10.00 ka in order to establish the minimum sea level during the Last Glacial Maximum (LGM) and to assess the validity, timing, and amplitude of meltwater pulses (so-called meltwater pulse 1A and 1B [MWP-1A and MWP-1B] events; ~13.80 and 11.30 ka, respectively) that are thought to have disturbed the general thermohaline oceanic circulation and, hence, global climate. Second, we will establish the SST variation accompanying the transgression at each transect. These data will allow us to examine the impact of sea level changes on reef growth, geometry, and biological makeup, especially during reef drowning events, and will help improve the modeling of reef development. The third major objective will be to identify and establish patterns of short-term paleoclimatic changes that are thought to have punctuated the transitional period between present-day climatic conditions following the LGM. We propose to quantify the variations of SSTs based on high-resolution isotopic and trace element analyses on massive coral colonies. When possible, we will try to identify specific climatic phenomena such as the El Nino–Southern Oscillation (ENSO) in the time frame prior to 10.00 ka