ALBERT

Description: The Cretaceous Heterohelix moremani (Cushman) was the only biserial planktonic foraminiferal species from its first appearance in the late Albian up to the Cenomanian/Turonian boundary. Within that time, it increased gradually in abundance relative to other planktonic foraminifera in five Circum-North Atlantic sections. It is generally rare in upper Albian sediments, common in most of the Cenomanian and very abundant in sediments representing the latest Cenomanian Oceanic Anoxic Event. Short-term variations on the overall abundance trend correlate with positive excursions in the bulk carbonate delta13C record. Maximum rain rates of H. moremani during OAE2 show that this species was an opportunist that did well in extreme conditions, but its overall distribution indicates that it is not necessarily a marker for very high palaeoproductivity environments. Stable oxygen and carbon isotope measurements on foraminiferal species indicate that H. moremani was a surface water dweller at least in part of its geographic range, but incorporated 13C out of equilibrium with ambient seawater. It is depleted in delta13C relative to other planktonic foraminifera, which is attributed to vital effects related to its opportunistic character.

Description: An Oligocene-basal Miocene (Zones P19-P22/N4) oxygen and carbon isotope stratigraphy is presented based on mixed benthic foraminiferal faunas and different species of planktic foraminifers from Eastem Indian Ocean ODP Hole 758A. Lack of covariance in planktic and benthic delta18O ratios indicates that many Oligocene sea level fluctuations, including the major fall at 30 Ma, are not of glacio-eustatic origin. Delta18O values suggest that test morphology is a poor indicator of preferred depth habitat of Oligocene planktic foraminifers. Also, some groups show considerable shifts in preferred depth habitat. Reconstructed surface water temperature was nearly constant during the investigated period, but even assuming a large Antarctic icecap, values are unrealistically low, 18°-20°C. Most likely, water masses with markedly different oxygen isotopic composition existed, this poses great problems for quantitative temperature reconstructions. Planktic foraminiferal species show similar delta13C ratios, indicating well-mixed surface waters. This contrasts with the strong oxygen isotopic gradient, which suggests well-stratified surface waters. During the Oligocene the difference in delta13C ratio between the average ratio of planktic foraminiferal species and the benthic foraminifers (Delta delta13C P-B') shows an inverse relation with the known circulation intensity record and thus with productivity. This Delta delta13C pattern is interpreted as being caused by the presence of younger bottom waters at this site during periods with lower temperatures and/or larger Antarctic ice sheets.

Description: About one third of the anthropogenic carbon dioxide (CO2) released into the atmosphere in the past two centuries has been taken up by the ocean. As CO2 invades the surface ocean, carbonate ion concentrations and pH are lowered. Laboratory studies indicate that this reduces the calcification rates of marine calcifying organisms, including planktic foraminifera. Such a reduction in calcification resulting from anthropogenic CO2 emissions has not been observed, or quantified in the field yet. Here we present the findings of a study in the Western Arabian Sea that uses shells of the surface water dwelling planktic foraminifer Globigerinoides ruber in order to test the hypothesis that anthropogenically induced acidification has reduced shell calcification of this species. We found that light, thin-walled shells from the surface sediment are younger (based on 14C and d13C measurements) than the heavier, thicker-walled shells. Shells in the upper, bioturbated, sediment layer were significantly lighter compared to shells found below this layer. These observations are consistent with a scenario where anthropogenically induced ocean acidification reduced the rate at which foraminifera calcify, resulting in lighter shells. On the other hand, we show that seasonal upwelling in the area also influences their calcification and the stable isotope (d13C and d18O) signatures recorded by the foraminifera shells. Plankton tow and sediment trap data show that lighter shells were produced during upwelling and heavier ones during non-upwelling periods. Seasonality alone, however, cannot explain the 14C results, or the increase in shell weight below the bioturbated sediment layer. We therefore must conclude that probably both the processes of acidification and seasonal upwelling are responsible for the presence of light shells in the top of the sediment and the age difference between thick and thin specimens.