Seaweed fails to prevent ocean acidification impact on foraminifera along a shallow-water CO2 gradient

Ocean acidification causes biodiversity loss, alters ecosystems, and may impact food security, as shells of small organisms dissolve easily in corrosive waters. There is a suggestion that photosynthetic organisms could mitigate ocean acidification on a local scale, through seagrass protection or seaweed cultivation, as net ecosystem organic production raises the saturation state of calcium carbonate making seawater less corrosive. Here, we used a natural gradient in calcium carbonate saturation, caused by shallow-water CO2 seeps in the Mediterranean Sea, to assess whether seaweed that is resistant to acidification (Padina pavonica) could prevent adverse effects of acidification on epiphytic foraminifera. We found a reduction in the number of species of foraminifera as calcium carbonate saturation state fell and that the assemblage shifted from one dominated by calcareous species at reference sites (pH 8.19) to one dominated by agglutinated foraminifera at elevated levels of CO2 (pH 7.71). It is expected that ocean acidification will result in changes in foraminiferal assemblage composition and agglutinated forms may become more prevalent. Although Padina did not prevent adverse effects of ocean acidification, high biomass stands of seagrass or seaweed farms might be more successful in protecting epiphytic foraminifera.

Isotope ratios of smectites, calcites and whole rocks from DSDP Legs 51 and 52, Atlantic Ocean

Delta180 and 87Sr/86Sr isotopic data from smectites, calcites, and whole rocks, together with published isotopic age determinations, alkali element concentration data and petrographic observations suggest a sequential model of ocean floor alteration. The early stage lasts about 3 m.y. and is characterized by palagonite and smectite formation, and solutions with a large basaltic component, increasing with temperature which varies from 15° to 80° C at DSDP site 418A. Most carbonates are deposited after this stage from solutions with a negligible basaltic Sr component and temperatures of 15° to 40° C. Water of seawater Sr and O isotopic composition is shown to percolate to at least 500 m into the basaltic basement. No evidence was found for continuing exchange of strontium or oxygen after 3 m.y.

Geochemistry of oceanic crust of the North Atlantic

87Sr/a6Sr ratios, Sr, K, Rb and Cs contents and the petrology of basalts and secondary phases recovered from deep basement drilling at DSDP Sites 332B, 417A, 417D and 418A show that the oceanic crust alters in four distinct stages characterized by formation of palagonite, smectite and calcite (Stages I, II and III, respectively). Stage IV represents the final compaction of the crust, including a dehydration of the crust without major chemical changes. Isotopic age determinations by Hart and Staudigel (1978, doi:10.1029/GL005i012p01009) and Richardson et al. (doi:10.1029/JB085iB12p07195), show that, at least for Atlantic-type crustal regimes, Stage I and II last for a maximum of 3 m.y., and Stage III lasts beyond Stage I and II, but is probably completed in significantly less than 10 m.y. Stage IV is long-lived and may still be active at 100 m.y. Stages I and II, the phase of halmyrolysis, include geochemically significant interactions between seawater and basalt for the upper 500 m of layer II and involve volumes of seawater containing a large basaltic component. Stage III solutions show evidence of less seawater-basalt interaction, at least to depths of 500 m; calcites deposited from these solutions have Sr isotopic ratios close to seawater values; but also have very low Sr/Ca ratios indicating a large basalt Ca component in the solutions. Smectite formation is the result of the interaction of seawater and basalt. The initial 87Sr/a6Sr ratios of smectites represent the Sr isotopic composition of the solution when the smectite is being formed. Thereafter, alkalies may be continuously added to interlayer positions in the smectite in order of decreasing hydration energy (Cs is more enriched than Rb, Rb more than K). The later-formed carbonates have very low alkali concentrations, and 87Sr/86Sr ratios identical to contemporaneous seawater. Therefore, since the alkali concentrations in a whole rock sample are affected by different alteration processes, the alkali concentrations alone are not reliable indicators of the degree of alteration.