Exploring aberrant bivalve shell ultrastructure and geochemistry as proxies for past sea water acidification

Abstract

Throughout much of Earth's history, marine carbonates have represented one of the most important geological archives of environmental change. Several pivotal events during the Phanerozoic, such as mass extinctions or hyperthermal events have recently been associated with ocean acidification. Nevertheless, well-defined geological proxies for past ocean acidification events are, at best, scarce. Here, experimental work explores the response of bivalve shell ultrastructure and isotope geochemistry (d 13 C, d 18 O and d 26 Mg) to stressful environments, in particular to sea water acidification. In this study, the common blue mussel, Mytilus edulis, was cultured (from early juvenile stages to one year of age) at four pH regimes (pH NBS 7Á2 to pH 8Á0). Shell growth rate and ultrastructure of mainly the calcitic portion of the shells were compared between experimental treatments. Specimens exposed to low-pH environments show patches of disordered calcitic fibre orientation in otherwise well-structured shells. Furthermore, the electron backscattered diffraction analyses reveal that, under acidified conditions, the c-axis of the calcite prisms exhibits a bimodal or multi-modal distribution pattern. Similar shell disorder patterns have been reported from mytilids kept under naturally acidified sea water conditions. In contrast, this study found no evidence that different pH regimes affect shell carbon, oxygen or magnesium isotope ratios. Based on these observations, it is proposed that: (i) stressful environments, in this case low sea water pH, predictably affect bivalve biomineralization patterns; and (ii) these findings bear potential as a novel (petrographic) proxy for ancient sea water acidification. An assessment of the applicability of these data to well-preserved fossil shell material from selected time intervals requires additional work.

Ocean acidification reduces the crystallographic control in juvenile mussel shells

“…M. edulis cultured for 12 months at pH 7.2 were also significantly reduced in shell length (Hahn et al, 2014), whereas earlier shell lengths of M. edulis did not show significant differences. The second generation juvenile mussels presented in this study produced significantly smaller at only 6 months in contrast to the findings of Hahn et al, 2014, this could suggest the inherited tolerance for aragonite production at lower pCO 2 at 550, 750 µatm could result in a trade off for shell length.…”

Geochemical and microstructural characterisation of two species of cool-water bivalves (<i>Fulvia tenuicostata</i> and <i>Soletellina biradiata</i>) from Western Australia

“…Freitas et al, 2006;Gillikin et al, 2005b;Lowenstam and Weiner, 1989) and environmental conditions at the time of deposition (e.g. Ferguson et al, 2011;Gazeau et al, 2010;Hahn et al, 2014;Heinemann, 2011;Henkes et al, 2013;Lowenstam and Weiner, 1989;Schöne et al, 2011). Molluscs are globally distributed and demonstrate sequential growth, thus providing high-resolution seasonal and sub-seasonal records of environmental conditions (e.g.…”

Mollusc and brachiopod skeletal hard parts: Intricate archives of their marine environment

“…Under extreme environmental stress, some aragonitic bivalves, such as Saxidomus sp. and Mytilus sp., reportedly form conically shaped or disordered calcite patches in their outer or inner shell layers, respectively, that broaden towards the following growth line (Carter et al ., ; Hahn et al ., , ). The outer shell surface is protected by the periostracum (Figs E and ) against dissolution and bioerosion.…”

“…A novel petrographic proxy approach is documented in Hahn et al (2014). These authors report on the effects of stressful environments, and particularly seawater acidification, on bivalve (Mytilus edulis) shell ultrastructure.…”

“…The research questions may or may not have relevance for sedimentologists or palaeoceanographers. Acknowledging the limitations of recent experimental organisms as analogues for their, in part extinct, fossil counterparts, it has been suggested that this approach has great potential (Lorens & Bender, 1980;Klein et al, 1996b;Hahn et al, 2014). Moreover, it appears that many marine organisms are conservative with respect to their biomineralization pathways.…”