The influence of skeletal micro-structures on potential proxy records in a bamboo coral

Abstract

Assessing the physicochemical variability of the deeper ocean is currently hampered by limited instrumental time series and proxy records. Bamboo corals (Isididae) form a cosmopolitan family of calcitic deep sea corals that could fill this information gap via geochemical information recorded in their skeletons. Here we evaluate the suitability of high-resolution chemical imaging of bamboo coral skeletons for temperature and nutrient reconstruction. The applied elemental mapping techniques allow to verify the suitability of the chosen transect on the sample section for paleo-reconstructions and enhance the statistical precision of the reconstruction. We measured Mg/Ca via electron microprobe at 1 μm resolution and Ba/Ca via laser ablation ICP-MS at 35 μm resolution in a historic specimen of Keratoisis grayi from the Blake Plateau off Eastern Florida. Long-term growth temperatures of 7.1 ± 3.4 °C (2SD) that are in agreement with recent ambient temperature range can be reconstructed from Mg/Ca ratios provided that anomalously Mg-enriched structural features around the central axis and isolated features related to tissue attachment are avoided for reconstruction. Skeletal Ba/Ca measurements reflect mean seawater barium [Ba]_SW concentrations ([Ba]_SW = 51 ± 24 nmol kg⁻¹ (2SD)), in agreement with instrumental data (47 nmol kg⁻¹). We show for the first time that Ba/Ca forms concentric structures in a bamboo coral skeleton section. Our investigations suggest that, while bamboo coral skeletons do record environmental parameters in their mean chemical composition, the magnitude of environmental variability reconstructed from high-resolution chemical maps exceeds that expected from instrumental time series. This necessitates additional investigation of the factors driving bamboo coral skeletal composition.

Introduction

Cold-water corals (CWCs) are receiving rising scientific interest as potential proxy recorders of a rarely observed environment. These animals can thrive in the deep sea – a habitat comprising the largest carbon reservoir in the ocean. Importantly, CWCs often inhabit hard substrates in environments where low sedimentation rates otherwise limit or even prohibit reconstructions from the sedimentary archive. Both global trends (such as changes in temperature or pH) and variable local oceanographic conditions (such as organic matter supply or shifting ocean currents) impact life in deep ocean ecosystems (Ruhl and Smith, 2004, Levin and Le Bris, 2015), but the spatial and temporal deficiency of instrumental records is a major hurdle to our understanding of longer term environmental conditions and oceanographic variability in the deep sea.

The lack of deep ocean instrumental data may be mitigated using detailed geochemical information from calcifying CWCs (e.g. Robinson et al., 2014). CWCs are surface- to deep-sea dwellers and have the potential to record environmental signals in their calcareous skeleton at up to sub-annual resolution (Sherwood and Risk, 2007). The high-magnesium calcite (HMC)-precipitating octocoral family Isididae – also called bamboo corals – lives in a depth range of less than 10 to more than 2000 m (Bostock et al., 2015, Thresher et al., 2016). They can therefore occur in waters well below the aragonite saturation horizon (Guinotte et al., 2006) where only a small fraction of aragonitic CWCs can survive (Cairns, 2007). Reconstructed bamboo coral radial growth rates range from 12–180 μm yr⁻¹ (Farmer et al., 2015b and references therein). Their calcitic ontogeny is sometimes characterised by growth rings (Noé and Dullo, 2006), and “spirallike” structures (Thresher and Neil, 2016), both of which can be observed by the naked eye. Geochemical analyses of these calcitic structures may therefore offer high-resolution environmental reconstructions, provided that ontogenetic effects on geochemistry can be understood. Because bamboo corals have long lifespans, sometimes in excess of 300 years (Andrews et al., 2009, Hill et al., 2011), splicing geochemical records of temporally overlapping specimens (Prouty et al., 2011) might allow reconstruction of (sub-)annual growth conditions over several hundreds of years.

Various elemental and isotope ratios have been analysed in both the organic nodes and calcareous internodes of bamboo corals (Robinson et al., 2014). For example, previous studies link internodal Mg/Ca to ambient water temperature (Thresher et al., 2004, Thresher et al., 2010, Thresher et al., 2016), δ¹¹B to ocean pH (Farmer et al., 2015a), and δ¹³C and δ¹⁸O to temperature (Hill et al., 2011, Kimball et al., 2014, Saenger et al., 2017). Additionally, bamboo coral Ba/Ca has been proposed as a nutrient proxy, with Ba/Ca indicating [Ba]_SW (LaVigne et al., 2011, Thresher et al., 2016, Serrato Marks et al., 2017) and/or [Si]_SW (Thresher et al., 2016). In the organic nodes, bulk δ¹⁵N was found to serve as an indicator of food source and ecosystem trophic dynamics (Sherwood et al., 2009, Hill et al., 2014). Furthermore, δ¹³C analyses on nodal organic material have been applied to reconstruct the δ¹³C of the exported surface ocean primary production (Schiff et al., 2014).

Although previous studies have generally demonstrated the potential of bamboo corals to record environmental conditions, calibrations established with bulk measurements may hamper fine-scale reconstructions of past environmental conditions (e.g. Aranha et al., 2014). In order to assess whether fine-scale environmental information can be extracted from single specimens, skeletal microstructures and their influence on the proxy record need to be investigated. In this study we quantify the spatial and microstructural chemical composition of bamboo corals using element mapping approaches via laser ablation ICP-MS, electron microprobe analysis, and confocal Raman microscopy. These techniques allow us to gain detailed information at high spatial resolution regarding elemental composition of bamboo coral internodes, growth structures and mineralogy. Specifically, we assess the ability of bamboo coral Mg/Ca and Ba/Ca to work as high-resolution temperature and nutrient proxies, respectively. To investigate the distribution of organic material, we mapped fluorescence and sulfur content via confocal Raman microscopy and electron microprobe, respectively. The employed techniques complement one another and shed light on how bamboo corals build their skeleton. Although LA-ICP-MS mapping has been applied as a promising new tool in paleoclimatic (Fietzke et al., 2015) and biomineralisation studies (Evans and Müller, 2013, Oppelt et al., 2017), fine-scale mapping and subsequent reconstruction of environmental parameters presented here are, to the best of our knowledge, the first application to octocorals.