Seasonal to decadal scale influence of environmental drivers on Ba/Ca and Y/Ca in coral aragonite from the southern Great Barrier Reef

Highlights

• Coral Ba/Ca and Y/Ca reflect terrestrial discharge over decadal time scales.

• Drought breaking floods result in increased coastal turbidity proxies.

• GBR sediment flux is decreased following intense flushing in preceding years.

Abstract

Extensive catchment modification since European settlement on the eastern coast of Australia results in poor coastal water quality, which poses a major threat for near shore coral communities in the iconic Great Barrier Reef (GBR). Long lived inshore corals have the potential to provide long-term temporal records of changing water quality both pre- and post-anthropogenic modification. However, water quality proxies require more study and validation of the robustness of coral-hosted geochemical proxies for a specific site is critical. This study investigated the long-term (1958–2010) influence of environmental drivers on high-resolution Ba/Ca and Y/Ca proxies obtained from Porites sp. coral from Great Keppel Island, southern GBR, Australia. Geochemical proxy records were influenced by environmental change on a seasonal to decadal scale. Although seasonal oscillations of Ba/Ca and Y/Ca were related to rainfall and discharge from the Fitzroy River catchment, some uncorrelated anomalous peaks were evident throughout the time series. Regardless, the behaviour of these proxies was significantly consistent over the longer time scale. Most long-term drought-breaking floods, including one that occurred in winter, resulted in significant increase in the targeted elemental ratios owing to higher terrigenous sediment flux to the near shore marine environment from a catchment with reduced groundcover. Following this intense flushing event, elemental ratios were reduced in subsequent wet periods as a result of less sediment being available for transport to coastal seawater. Ba/Ca and Y/Ca proxies can be valuable tools in reconstructing multiyear variations in terrestrial runoff and associated inshore water quality. As these proxies and their regional and local controls are better understood they will aid our understanding of how reefs have responded and may respond to changing water conditions.

Introduction

Coral reefs are integral parts of marine ecosystems and support immense biodiversity. Nearshore reefs located close to river mouths have been subjected to high rates of disturbance in many parts of the world. In addition to the variety of changing global climate parameters (e.g., ocean acidification, tropical cyclones, and CO₂ induced rise in sea surface temperature (SST)) that affect offshore reefs, inshore reef systems face significant threats from more localized anthropogenic inputs (e.g., pollution from increased terrestrial sediment and nutrient loads, dredging and dumping, agri-chemicals and industrial toxins and metals) (Brodie, 2014; Brodie et al., 2013; De'ath et al., 2012; Hughes, 1994; Hughes et al., 2003; Pandolfi et al., 2003). Hence, although inshore reef communities tend to be adapted to higher turbidity environments (Larcombe et al., 2001; Perry and Smithers, 2010; Perry et al., 2012), they are increasingly at risk from anthropogenic pressures. In order to protect such reefs and potentially predict their future responses to changing water conditions, it is critical to develop specific environmentally linked proxies to extend our understanding of water quality before instrumental records (De'ath and Fabricius, 2010; De'ath et al., 2012; Greer et al., 2009; Wooldridge and Done, 2009).

Annually banded coral skeletons can record a variety of environmental data in marine settings and have proven to be useful archives of various proxies for high-resolution temporal reconstruction of past environment and climate. The great value of coral archives resides in their wide geographic distribution, long temporal span (up to a thousand years) and their resistance to erosion and breakage (Fleitmann et al., 2007; Veron, 2000). For example, elemental proxies in coral skeletons have been used commonly to reconstruct SST (Beck et al., 1992; Duprey et al., 2012; Felis et al., 2009; Gagan et al., 1998; Jimenez et al., 2018; Kawakubo et al., 2017; McCulloch et al., 1994; Min et al., 1995; Mitsuguchi et al., 1996; Sadler et al., 2016; Wang et al., 2018), salinity (Shen and Dunbar, 1995), terrestrial runoff (Alibert et al., 2003; Brenner et al., 2017; Jiang et al., 2017; Jupiter et al., 2008; McCulloch et al., 2003; Prouty et al., 2010; Sinclair, 2005), precipitation (Grove et al., 2013; Horta-Puga and Carriquiry, 2012; Moyer et al., 2012) and upwelling (Lea et al., 1989; Montaggioni et al., 2006; Reuer et al., 2003; Watanabe et al., 2017). Spatio-temporal variations in coral trace element proxies have been used increasingly to monitor coastal marine water quality affected by anthropogenic activities (e.g., land use change, dredging and dumping, sewage discharge, mining and oil pollution) (Fallon et al., 2002; Fleitmann et al., 2007; Nguyen et al., 2013). A current review concluded that among numerous elemental ratios, Ba/Ca and Y/Ca proxies have been shown to have close links to sediment loads delivered by river systems to inshore marine environments (Saha et al., 2016). However, some studies reported anomalous peaks of these proxies that are decoupled from terrestrial runoff and other known events, suggesting that these proxies are not solely controlled by runoff. Other factors (e.g., upwelling, regional hydrodynamics, biological activities, sediment resuspension and groundwater seeps) also can have significant influence on their temporal records (Alibert et al., 2003; Lea et al., 1989; Lewis et al., 2012; Montaggioni et al., 2006; Sinclair, 2005). Hence, the reliability of these elemental ratios as runoff proxies should be validated before interpreting their extended records in a particular region.

Corals can preserve a record of changes in water quality because many trace elements are incorporated into the coral's aragonite skeleton in close proportion to their abundance in ambient seawater (LaVigne et al., 2016; Shen et al., 1987). Thus their concentrations can reflect seasonal, annual, and decadal variability in water quality where the corals grew. The mechanisms for incorporation of trace elements into coral skeleton from ambient seawater have been widely studied (Allison, 1996; Allison and Finch, 2004; Amiel et al., 1973; Barnard et al., 1974; Brown et al., 1991; Mitterer, 1978; Shen et al., 1991; St John, 1974), but they are not fully understood owing in part to the effects of various biological factors (so-called “vital effects”) and subsequent, but early, diagenetic alteration. Trace elements in coral skeleton can substitute for Ca in the crystal lattice, be trapped in pore spaces, adsorbed on exposed skeletal surfaces, or form complexes with organic matter (for details, Saha et al., 2016). Hence, interpretation of skeletal elemental proxies can be complicated, but there is strong empirical support for their use in environmental archives (Beck et al., 1992; Gagan et al., 2000; Gagan et al., 1998; Lewis et al., 2012; McCulloch et al., 2003).

Records of Ba/Ca and Y/Ca proxies in high-latitude inshore coral reefs of the southern GBR are basically absent from the literature (Fig. 1a, b), but such data are required in both space and time in order to better develop their links with specific environmental drivers. In this study, we present Ba/Ca and Y/Ca proxies in core recovered from a Porites coral colony with 53 years of recorded growth (from December 1957 to July 2010) from near-shore Great Keppel Island, southern GBR. The Keppel Islands are located north of the mouth of the Fitzroy River, which drains a catchment that has been extensively modified as a result of land clearing for grazing in both coastal and inland regions as well as urban development since European colonisation in the mid-19th century. Increased sediment loads from the catchment have decreased local water quality causing smothering of inshore corals and seagrass communities (GBRMPA, 2013). Here, the integrity of high-resolution Ba/Ca and Y/Ca proxies were tested against environmental drivers (Fitzroy River discharge and rainfall) over seasonal, annual and decadal time scales. Our results should improve understanding of these proxies in this particular region, and thus allow better interpretation of the impact of key environmental drivers in modulating the time series geochemical data. That improved understanding should have application for interpreting local records in older corals, prior to instrumental measurements, and potentially the interpretation of data from other regions.