Abigail M. Smith

Effects of early sea-floor processes on the taphonomy of temperate shelf skeletal carbonate deposits

Abstract Cool-water shelf carbonates differ from tropical carbonates in their sources, modes, and rates of deposition, geochemistry, and diagenesis. Inorganic precipitation, marine cementation, and sediment accumulation rates are absent or slow in cool waters, so that temperate carbonates remain longer at or near the sea bed. Early sea-floor processes, occurring between biogenic calcification and ultimate deposition, thus take on an important role, and there is the potential for considerable taphonomic loss of skeletal information into the fossilised record of cool-water carbonate deposits. The physical breakdown processes of dissociation, breakage, and abrasion are mediated mainly by hydraulic regime, and are always destructive. Impact damage reduces the size of grains, removes structure and therefore information, and ultimately may transform skeletal material into anonymous particles. Abrasion is highly selective amongst and within taxa, their skeletal form and structure strongly influencing resistance to mechanical breakdown. Dissolution and precipitation are the end-members of a two-way chemical equilibrium operating in sea water. In cool waters, inorganic precipitation is rare. There is conflicting opinion about the importance of diagenetic dissolution of carbonate skeletons on the temperate sea floor, but test maceration and early loss of aragonite in particular are reported. Dissolution may relate to undersaturated acidic pore waters generated locally by a combination of microbial metabolisation of organic matter, strong bioturbation, and oxidation of solid phase sulphides immediately beneath the sea floor in otherwise very slowly accumulating skeletal deposits. Laboratory experiments demonstrate that surface-to-volume ratio and skeletal mineralogy are both important in determining skeletal resistance to dissolution. Biological processes on the sea floor include encrustation and bioerosion. Encrustation, a constructive process, may be periodic or seasonal, and can be reversed. It produces both information and material. Bioerosion, in contrast, is destructive and permanent. In temperate areas bioerosion may destroy even very large shells during their long residence at the sea floor, on the order of hundreds to thousands of years. Overall, processes on the temperate sea floor may combine to destroy more carbonate than they produce, and the preservation potential of temperate shelf carbonate into the rock record may be significantly affected. Where preservation does occur in such a destructive regime, the effects of early sea-floor processes will be key determinants of the deposit, resulting in a “taphofacies” characteristic of temperate shelf carbonate sediments.

Skeletal carbonate mineralogy of New Zealand bryozoans

Abstract On many cool-water carbonate shelves, both modern and ancient, bryozoans are the dominant sediment contributor. Because skeletal carbonate mineralogy can be related to both taxonomic and environmental controls, and has important implications for understanding the geochemistry and diagenesis in carbonate deposits, it is highly relevant to develop a comprehensive database for bryozoan skeletal mineralogy. This X-ray diffraction study of modern New Zealand bryozoans more than doubles the published data available on bryozoan mineralogy and allows for statistical analysis of variability in mineralogy, particularly the hitherto unknown background variation within individuals and within populations. Such a database allows evaluation of differences among populations, taxa, and growth forms. The 412 analyses of 49 bryozoan species fall into four mineralogical categories: (a) 61% are single calcite, which ranges widely from low to high wt% MgCO3 varieties; (b) 6% (in the genera Cellaria and Macropora) comprise dual calcites, a dominant one low in Mg ( X =1.8 wt% MgCO3) and the other much higher ( X =9.0 wt% MgCO3); (c) 27% are mainly calcite with some aragonite; and (d) 6% are mainly aragonite with some calcite. The bimineralic calcite–aragonite skeletons can arise from different processes, including biologically mediated changes in precipitation and diagenetic alteration following death. There is small but significant variation in Mg content in calcite both within colonies (Mg increasing with age) and within populations, for a total of within-species variance of 0.526. We found, however, more variation among species in both calcite/aragonite ratio and Mg content than that within species. Whereas some genera and families are reasonably consistent mineralogically (e.g., low-Mg calcite cyclostomes such as Telopora buski), others are highly variable (e.g., Chaperia acanthina). Among the higher taxa, cyclostomes tend to be entirely or mainly calcite with a low to moderate range of MgCO3 ( X =2.1 wt%), whereas cheilostomes are far more variable, with anascans ( X =4.1 wt%) having a lower MgCO3 content than ascophorans ( X =5.6 wt%). All dual-calcite species are anascans in the Calloporoidea. The proportions of aragonite and of Mg in calcite are greater in evolutionarily younger taxa, at least at the higher levels. In relation to growth forms, erect rigid varieties are calcitic except for several robust-branching genera such as Adeonellopsis. Encrusting forms, particularly unilaminar sheets, are highly variable and contain most of the bimineralic calcite–aragonite species. Erect flexible forms are either single or dual calcite, and extremely variable in Mg content. Free-living or vagrant forms are usually aragonitic. In relation to latitude and temperature variation, few species show any consistent or significant mineralogical trends, although Schizoporella unicornis deposits increasing amounts of aragonite at lower (warmer) latitudes. Mecynoecia purpurascens includes more (but still low) Mg in its calcite skeleton with increasing latitude, a reversal of the Mg–temperature relationship noted in several other skeletal carbonate producers.

Influence of the small intertidal seagrass Zostera novazelandica on linear water flow and sediment texture

Abstract The influence of a small intertidal seagrass, Zostera novaz.elandica (Setchell), on tidal current velocities and sediment texture was studied at Harwood, Otago Harbour, New Zealand. Tidal current velocities were substantially reduced inside the Z. novazelandica patch compared to velocities above the patch (3.7 times greater) and outside it (2.5 times greater), causing a low‐flow environment among the 12‐cm‐tall seagrass. Current velocities combined over one tidal cycle occupied a much wider range outside ( 1.2‐A.b cms‐1) and above ( 1.9–7.1 cm s‐1) the seagrass patch than inside (0.1–1.8 cm s‐1) indicating slower and less variable water flow. Suspended mud (<0.063 mm) settled to the bottom in this low‐energy environment and was protected from resuspension by seagrass cover, indicated by significantly elevated amounts of mud inside the seagrass patch (1.1%) compared to outside (0.4%). These results indicate that Z. novazelandica may reduce water flow and accumulate finer sediment sizes to a lesser extent than larger subtidal seagrass species.

Bryozoans as southern sentinels of ocean acidification: a major role for a minor phylum

Rapid anthropogenic production of CO2 has driven the carbonate chemistry of the sea, causing lowered pH in surface waters. Increasingly, scientists are called on to study ocean acidification and its effects. The ‘minor’ phylum Bryozoa shows considerable potential in understanding temperate southern hemisphere shelf carbonate dynamics, thus complementing tropical studies based mainly on corals. Lowered pH affects skeletons differently depending on their composition, but skeletons are even more strongly affected by morphology. Different bryozoans will manifest the effects of acidification at different times, thus some particularly vulnerable species may act as ‘canaries’ providing an early warning for some shelf communities, such as bryozoan-dominated thickets. A carbonate budget based on several studies of the bryozoan Adeonellopsis in Doubtful Sound, New Zealand, shows that increasing dissolution pressure in cool temperate environments dramatically reduces sediment accumulation rates. Bryozoan shelf carbonate sediments, which blanket the southern shelves of New Zealand and Australia, may serve as biological saturometers, monitoring the effects of acidification over shelf depths. Whether acting as canaries, models or sentinels, bryozoans have great potential to provide insight into the next global challenge: ocean acidification.

Growth and carbonate production by Adeonellopsis (Bryozoa: Cheilostomata) in Doubtful Sound, New Zealand

The erect arborescent bryozoan Adeonellopsis sp. is an important component of the attached fauna on rock faces in Doubtful Sound, New Zealand. A program of marking and harvesting, radiocarbon dating, and morphometric study was undertaken to determine age, growth rate, and carbonate production rate of these colonies. Data from 40 branches on each of five colonies show a growth rate of 6.9 mm/yr in branch length. Colony growth rate varied, with 71% of growth (5 mm per branch) occurring from mid-summer and to mid-winter, and 29% (2 mm per branch) from midwinter to late summer. Since proximal secondary thickening is common in adeoniform species, and occurs in Adeonellopsis, additional carbonate may be precipitated annually by this means. The largest colonies found in Doubtful Sound, some up to 30 cm in diameter, may be as much as 20 yr old, and precipitate calcium carbonate at a rate of 24 g CaCO3/yr. At Bauza Island, where our study was carried out, population densities of one large colony/m produced carbonate at a rate of 24 g CaCO3/m 2 /yr; maximum theoretical density could produce carbonate at 1042 g CaCO3/m 2 / yr. Carbonate produced at these rates would accumulate in sediments at 4^174 cm/kyr, reasonable rates for temperate carbonates. Adeonellopsis provides substrate for epizoa and hiding places for motile organisms. They form a potentially important fiord microhabitat, and their longevity allows both more ephemeral organisms and young longer-lived colonies to grow under their protection. fl 2001 Elsevier Science B.V. All rights reserved.

Mineralogical Variation in Shells of the Blackfoot Abalone, Haliotis iris (Mollusca: Gastropoda: Haliotidae), in Southern New Zealand

The New Zealand blackfoot abalone, Haliotis iris Gmelin, is among the few gastropods that precipitate both calcite and aragonite in their shells. The location, composition, and thickness of these mineral layers may affect color, luster, and strength of the shell, which is locally important in jewelry manufacture. Skeletal mineralogy and shell structure of H. iris from three southern New Zealand locations were determined using X-ray diffractometry, scanning electron micrography, and mineral staining. In H. iris an outer calcitic layer is separated from an inner aragonitic surface by both calcified and noncalcified organic layers running longitudinally through the shell. Skeletal mineralogy within individual shells varies from 29 to 98% aragonite, with older shell having significantly higher aragonite content than young sections. Variation within populations ranges from 40 to 98% aragonite, and among three populations from 34 to 98% aragonite. Shell thickness, too, varies within individual shells from 0.2 to 4.2 mm, with a significant positive relationship with age. Withinpopulation variation in shell thickness ranges from 2.1 to 5.4 mm, with no significant difference in shell thickness variation among populations. The high degree of variability within and among individual shells suggests that it is essential to test replicate samples from individual mollusk shells, especially when they have complex bimineral structure.

Phylomineralogy of the Coralline red algae: Correlation of skeletal mineralogy with molecular phylogeny

The coralline algae in the orders Corallinales and Sporolithales (subclass Corallinophycidae), with their high degree of mineralogical variability, pose a challenge to projections regarding mineralogy and response to ocean acidification. Here we relate skeletal carbonate mineralogy to a well-established phylogenetic framework and draw inferences about the effects of future changes in sea-water chemistry on these calcified red algae. A collection of 191 coralline algal specimens from New Zealand, representing 13 genera and 28 species, included members of three families: Corallinaceae, Hapalidiaceae, and Sporolithaceae. While most skeletal specimens were entirely calcitic (range: 73-100 wt.% calcite, mean 97 wt.% calcite, std dev=5, n=172), a considerable number contained at least some aragonite. Mg in calcite ranged from 10.5 to 16.4 wt.% MgCO(3), with a mean of 13.1 wt.% MgCO(3) (std dev=1.1, n=172). The genera Mesophyllum and Lithophyllum were especially variable. Growth habit, too, was related to mineralogy: geniculate coralline algae do not generally contain any aragonite. Mg content varied among coralline families: the Corallinaceae had the highest Mg content, followed by the Sporolithaceae and the Hapalidiaceae. Despite the significant differences among families, variation and overlap prevent the use of carbonate mineralogy as a taxonomic character in the coralline algae. Latitude (as a proxy for water temperature) had only a slight relationship to Mg content in coralline algae, contrary to trends observed in other biomineralising taxa. Temperate magnesium calcites, like those produced by coralline algae, are particularly vulnerable to ocean acidification. Changes in biomineralisation or species distribution may occur over the next few decades, particularly to species producing high-Mg calcite, as pH and CO(2) dynamics change in coastal temperate oceans.

Warming Influences Mg2+ Content, While Warming and Acidification Influence Calcification and Test Strength of a Sea Urchin

We examined the long-term effects of near-future changes in temperature and acidification on skeletal mineralogy, thickness, and strength in the sea urchin Tripneustes gratilla reared in all combinations of three pH (pH 8.1, 7.8, 7.6) and three temperatures (22 °C, 25 °C, 28 °C) from the early juvenile to adult, over 146 days. As the high-magnesium calcite of the echinoderm skeleton is a biomineral form highly sensitive to acidification, and influenced by temperature, we documented the MgCO3 content of the spines, test plates, and teeth. The percentage of MgCO3 varied systematically, with more Mg2+ in the test and spines. The percentage of MgCO3 in the test and teeth, but not the spines increased with temperature. Acidification did not change the percentage MgCO3. Test thickness increased with warming and decreased at pH 7.6, with no interaction between these factors. In crushing tests live urchins mostly ruptured at sutures between the plates. The force required to crush a live urchin was reduced in animals reared in low pH conditions but increased in those reared in warm conditions, a result driven by differences in urchin size. It appears that the interactive effects of warming and acidification on the Mg2+ content and protective function of the sea urchin skeleton will play out in a complex way as global climatic change unfolds.

Effect of Ocean Acidification and pH Fluctuations on the Growth and Development of Coralline Algal Recruits, and an Associated Benthic Algal Assemblage

Coralline algae are susceptible to the changes in the seawater carbonate system associated with ocean acidification (OA). However, the coastal environments in which corallines grow are subject to large daily pH fluctuations which may affect their responses to OA. Here, we followed the growth and development of the juvenile coralline alga Arthrocardia corymbosa, which had recruited into experimental conditions during a prior experiment, using a novel OA laboratory culture system to simulate the pH fluctuations observed within a kelp forest. Microscopic life history stages are considered more susceptible to environmental stress than adult stages; we compared the responses of newly recruited A. corymbosa to static and fluctuating seawater pH with those of their field-collected parents. Recruits were cultivated for 16 weeks under static pH 8.05 and 7.65, representing ambient and 4× preindustrial pCO2 concentrations, respectively, and two fluctuating pH treatments of daily x~ = 8.05 (daytime pH = 8.45, night-time pH = 7.65) and daily x~ = 7.65 (daytime pH = 8.05, night-time pH = 7.25). Positive growth rates of new recruits were recorded in all treatments, and were highest under static pH 8.05 and lowest under fluctuating pH 7.65. This pattern was similar to the adults’ response, except that adults had zero growth under fluctuating pH 7.65. The % dry weight of MgCO3 in calcite of the juveniles was reduced from 10% at pH 8.05 to 8% at pH 7.65, but there was no effect of pH fluctuation. A wide range of fleshy macroalgae and at least 6 species of benthic diatoms recruited across all experimental treatments, from cryptic spores associated with the adult A. corymbosa. There was no effect of experimental treatment on the growth of the benthic diatoms. On the community level, pH-sensitive species may survive lower pH in the presence of diatoms and fleshy macroalgae, whose high metabolic activity may raise the pH of the local microhabitat.

Subtidal Galeolaria hystrix (Polychaeta: Serpulidae) reefs in Paterson Inlet, Stewart Island, New Zealand

Abstract Serpulid patch reefs of Galeolaria hystrix Mörch, 1863 were found in water depths of 9–16 m in Big Glory Bay, Paterson Inlet, Stewart Island, and here we report preliminary studies of these important habitat‐formers. This is the first observation of this species in subtidal patch reefs; 114 reefs were noted in a survey of 28 000 m2. Most reefs were 1–5 m in diameter, and up to 1.5 m high. Up to 65% of the serpulid tubes were occupied by living G. hystrix during a mid‐winter diving survey; 64% of reefs observed were whole, whereas 36% were broken or dead. Radiometric dating of a basal specimen of reef carbonate showed it to be less than 50 years old. Production of high‐Mg calcite, ranging from 9 to 11 wt% MgCO3, by G. hystrix may be as much as 11 kg CaCO3 m‐2y‐1, but was not reflected in surrounding sediments, which were dominantly terrigenous muds. A rich reef fauna, both sessile and motile, was associated with the reefs. Further study of these unusual temperate reefs is strongly recommended.