Holocene microbialite geochemistry records > 6000 years of secular influence of terrigenous flux on water quality for the southern Great Barrier Reef

Abstract

Anthropocene climate change and water quality degradation represent unprecedented challenges to modern coral reefs. Although declining reef health after European colonization is well documented around the world and increased terrigenous sediment flux is known to have terminated deglacial reefs in the Great Barrier Reef (GBR), longer-term patterns of water quality are poorly understood. Here we present the first direct proxy-based Holocene water quality reconstruction for any reef. The unique geochronological framework provided by cores from Heron and One Tree reefs (offshore, southern GBR) allowed reconstruction of offshore water quality from 8200 to 1800 years before present (BP) using centennially resolved microbialite-based geochemical proxies. Terrigenous sediment-sourced trace elements were measured in microbialites from a well dated succession of reef rock and in paleosol (ancient soil) formed at the Pleistocene-Holocene unconformity. Microbialite-hosted rare earth element and yttrium distributions (e.g., Nd/YbSN = 0.36; Y/Ho = 57) are consistent with precipitation from shallow oxygenated seawater but show a non-linear trend through the Holocene with distinct intervals of higher and lower terrigenous influence relative to average values. Immediately following reef initiation (>8300 years ago) our data suggest increasing terrigenous influence by 8000 ka. Surrounding reef seawater became less affected by terrigenous runoff from ~7000 years ago, but showed marked mid-Holocene variability related to regional climatic factors. Major fluctuations between intervals of high and low relative terrigenous influence correlate well with particular regional and more global climate records. These include local relative sea level fluctuations, fluctuations in Indian-Australian Summer Monsoon (IASM) strength, and dampened El Niño Southern Oscillation (ENSO) frequency corresponding to greater terrigenous influence in the southern GBR at ~7.0, 5.4, and 2.7 ka BP. Water quality then improved significantly after 3200 years BP. More broadly, it is well established that water quality has a major effect on reefs and reef communities, but for past reef history, inferences about water quality are commonly highly speculative. Reefal microbialite geochemistry provides an independent, high-quality proxy for ambient water quality that can be used to directly compare contemporaneous reef growth dynamics and ecological shifts to changing water quality. The high concentrations of trace elements in reefal microbialites, relative to other marine carbonates, provide a very robust, if time averaged, proxy for investigating ancient seawater chemistry, even in offshore reefs, such as Heron Reef. At the same time, the proxy provides a new independent data set for that may aid interpretation and model of climate change relevant to reef growth at centennial to millennial scales. As reefal microbialites are common in many global reef systems, where associated with high quality dating, they may provide useful proxies for investigating secular changes in water quality and associated climatic drivers at various temporal scales in other regions of the world.