No abstract

Anthropogenically-modulated reductions in pH, termed ocean acidification, could pose a major threat to the physiological performance, stocks, and biodiversity of calcifiers and may devalue their ecosystem services. Recent debate has focussed on the need to develop approaches to arrest the potential negative impacts of ocean acidification on ecosystems dominated by calcareous organisms. In this study, we demonstrate the role of a discrete (i.e. diffusion) boundary layer (DBL), formed at the surface of some calcifying species under slow flows, in buffering them from the corrosive effects of low pH seawater. The coralline macroalga Arthrocardia corymbosa was grown in a multifactorial experiment with two mean pH levels (8.05 ‘ambient’ and 7.65 a worst case ‘ocean acidification’ scenario projected for 2100), each with two levels of seawater flow (fast and slow, i.e. DBL thin or thick). Coralline algae grown under slow flows with thick DBLs (i.e., unstirred with regular replenishment of seawater to their surface) maintained net growth and calcification at pH 7.65 whereas those in higher flows with thin DBLs had net dissolution. Growth under ambient seawater pH (8.05) was not significantly different in thin and thick DBL treatments. No other measured diagnostic (recruit sizes and numbers, photosynthetic metrics, %C, %N, %MgCO3) responded to the effects of reduced seawater pH. Thus, flow conditions that promote the formation of thick DBLs, may enhance the subsistence of calcifiers by creating localised hydrodynamic conditions where metabolic activity ameliorates the negative impacts of ocean acidification.

The stable oxygen isotope composition (δ 18 O) of a precipitated carbonate depends mainly on the isotope composition, salinity, and temperature of the host fluid, whereas the stable carbon isotope composition (δ 13 C) reflects the source of CO 2 for precipitation, such as meteoric or sea water, shell dissolution, or various biochemical origins, including microbial oxidation of organic matter and methane. Despite the potentially complex array of controls, natural waters tend to show a characteristic range of isotope values which in turn are mimicked or tracked by the carbonate minerals precipitated from them. Consequently, plots of δ 18 O versus δ 13 C for carbonate materials can help identify their depositional and/or diagenetic environment(s).Here we compile isotope results for about 800 samples of carbonate skeletons, bulk sediments, fossils, limestones, cements, concretions, and veins spanning a range of ages (latest Cretaceous to modern) and locations in temperateregion New Zealand. Despite some overlap, the isotope values for 21 defined categories of carbonate materials tend to group in discrete regions of the δ 18 O-δ 13 C diagram, thereby providing important insights about their origin. New Zealand modern skeletal shelf carbonates plot in a distinctly different field from their tropical counterparts because of their heavier δ 18 O and narrower range of δ 13 C values, in accord with their nontropical bryomol skeletal facies. Cements in New Zealand temperate limestones are predominantly of burial or, more rarely, meteoric origin, but their nontropical heritage based solely on isotope composition becomes blurred and eventually lost as diagenesis proceeds. In common with many other global examples, siderite, calcite, and dolomite concretions have developed during shallow burial in a range of freshwater to marine depositional settings, the carbon originating mainly from early diagenetic, bacterially mediated reactions involving the decomposition of organic matter in bottom sediments. A summary δ 18 O-δ 13 C diagram showing mean and standarddeviation isotope values for the various New Zealand carbonate categories will form a basis for comparing and interpreting other carbonate materials and deposits. G95007

No abstract