Christopher W. Smart

A benthic foraminiferal proxy of pulsed organic matter paleofluxes

In the temperate open ocean of the modern northeast Atlantic, the spring bloom of phytoplankton leads to a seasonal pulse of detrital organic material (phytodetritus) to the ocean floor. Opportunistic benthic foraminifera rapidly colonise this food resource, producing large numbers of individuals whose tests are ultimately added to the sediment. One of these taxa,Epistominella exigua, shows periodic peaks in abundance in the fossil record at many open ocean sites. Previously, such peaks have been commonly interpreted to result from changes in physicochemical properties of bottom water mass and thus deep-sea circulation. The main purpose of this paper is to propose thatE. exigua may be used as a proxy of pulsed organic matter inputs to the deep ocean and therefore as an indicator of relative changes in productivity.

Late Pleistocene tephrochronology of marine sediments adjacent to Montserrat, Lesser Antilles volcanic arc

The recent history of the Soufrière Hills volcano, Montserrat, Lesser Antilles volcanic arc, is deduced using data obtained from a submarine core collected in 2002. The core contains concentrations of ash and several tephra layers, which are identified by the abundance of glass shards, dense and poorly vesiculated particles, and scoria. The tephra layers have been dated using micropalaeontology and stable isotope stratigraphy. Tephra layers in a marine sediment core off the coast of Montserrat record the volcanic history of South Soufrière Hills–Soufrière Hills volcano back to 250 ka. Eight layers are composed of dense juvenile ash related to dome eruptions, five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land. Six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions. A marker sequence of basalt tephra layers is dated at 124–147 ka and is correlated with construction of the South Soufrière Hills basaltic stratocone. Pelagic sediments between the main tephra layers have low abundances of volcanogenic components (<15%) and suggest long periods (c. 104 years) of dormancy or low activity.

Modern seawater acidification: the response of foraminifera to high-CO2 conditions in the Mediterranean Sea

Abstract: The seas around the island of Ischia (Italy) have a lowered pH as a result of volcanic gas vents that emit carbon dioxide from the sea floor at ambient seawater temperatures. These areas of acidified seawater provide natural laboratories in which to study the long-term biological response to rising CO2 levels. Benthic foraminifera (single-celled protists) are particularly interesting as they have short life histories, are environmentally sensitive and have an excellent fossil record. Here, we examine changes in foraminiferal assemblages along pH gradients at CO2 vents on the coast of Ischia and show that the foraminiferal distribution, diversity and nature of the fauna change markedly in the living assemblages as pH decreases.

The search for the origin of the planktic Foraminifera

Planktic Foraminifera are an extremely abundant, important and successful group of marine protists. They are particularly useful in reconstructing past environments and for biostratigraphic dating. Despite their importance, the origin of the group is uncertain. Previous work has suggested that they evolved from a benthic ancestor during the Triassic or, perhaps, the Mid-Jurassic (?Bajocian), but a reason for their origination has remained unclear. Here, we present evidence from the Toarcian (early Jurassic) of NW Europe that the origin of the planktic Foraminifera may have been one of the results of the early Toarcian oceanic anoxic event. This event appears to have been associated with a massive dissociation of gas hydrates and other, perhaps related, water chemistry changes.

Surviving mass extinction by bridging the benthic/planktic divide

Evolution of planktic organisms from benthic ancestors is commonly thought to represent unidirectional expansion into new ecological domains, possibly only once per clade. For foraminifera, this evolutionary expansion occurred in the Early–Middle Jurassic, and all living and extinct planktic foraminifera have been placed within 1 clade, the Suborder Globigerinina. The subsequent radiation of planktic foraminifera in the Jurassic and Cretaceous resulted in highly diverse assemblages, which suffered mass extinction at the end of the Cretaceous, leaving an impoverished assemblage dominated by microperforate triserial and biserial forms. The few survivor species radiated to form diverse assemblages once again in the Cenozoic. There have, however, long been doubts regarding the monophyletic origin of planktic foraminifera. We present surprising but conclusive genetic evidence that the Recent biserial planktic Streptochilus globigerus belongs to the same biological species as the benthic Bolivina variabilis, and geochemical evidence that this ecologically flexible species actively grows within the open-ocean surface waters, thus occupying both planktic and benthic domains. Such a lifestyle (tychopelagic) had not been recognized as adapted by foraminifera. Tychopelagic are endowed with great ecological advantage, enabling rapid recolonization of the extinction-susceptible pelagic domain from the benthos. We argue that the existence of such forms must be considered in resolving foraminiferal phylogeny.

A Model of early to middle Miocene Deep Ocean circulation for the Atlantic and Indian Oceans

Abstract Tethyan Outflow Water (TOW), characterized by high benthic δ13C values, was a component of the early Miocene deep water mass of the Atlantic (c. 20 and c. 13 Ma) and Indian (c. 20 and c. 14.5 Ma) Oceans. Fluctuations in temperature, salinity and quantity of dissolved oxygen coincided with high abundances of infaunal, smooth-walled bolivinids within the Atlantic and Indian Ocean TOW between 19.5 and 16.5 Ma. High δO values recorded in the Atlantic water mass may have originated, in part, from evaporation and the formation of warm, saline water within the Tethys. The southward flowing Atlantic TOW at depths of 2–4.5 km enhanced meridional heat transport. It was overlain by Northern Component Water (NCW) with a lower δC signal. TOW and NCW were replaced by southward flowing North Atlantic Deep Water at c. 13 Ma when cold dense Norwegian—Greenland Sea water flowed across the Greenland—Scotland Ridge. Indian Ocean TOW was restricted laterally by the Chagos—Lacadive Ridge, and vertically (1.3 to 3 km) by overlying intermediate water, characterized by the occurrence of coarse-walled bolivinids, and underlying deep water characterized by high abundances (>10%) of Nuttallides umboniferus. The coarse-walled bolivinid and ‘N. umboniferus’ faunas were associated with lower temperature, salinity and higher oxygen concentrations. Tethyan outflow to the northwest Indian Ocean ended at c. 14.5 Ma, probably in response to the closure of a gateway, and resulted in a two-component deep water mass structure. The coeval increase in the relative abundances of N. umboniferus at Indian Ocean Sites 237 (c. 2 km) and 710 (c. 3.5 km), at c. 13 Ma, probably records the impact of this change on the nature of Indian Ocean deep water at depths between 2 and 4 km. The termination of the Atlantic and, to a lesser extent, Indian Ocean TOW contributed to global cooling and to the expansion of the Antarctic Ice Sheet by limiting the meridional heat transport.

Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California, Mexico.

Extensive CO2 vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO2 leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74-207m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO2 vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.

The enigma of early Miocene biserial planktic foraminifera

Small biserial foraminifera were abundant in the early Miocene (ca. 18.9–17.2 Ma) in the eastern Atlantic and western Indian Oceans, but absent in the western equatorial Atlantic Ocean, Weddell Sea, eastern Indian Ocean, and equatorial Pacific Ocean. They have been assigned to the benthic genus Bolivina, but their high abundances in sediments without evidence for dysoxia could not be explained. Apertural morphology, accumulation rates, and isotopic composition show that they were planktic (genus Streptochilus). Living Streptochilus are common in productive waters with intermittent upwelling. The widespread early Miocene high Streptochilus abundances may reflect vigorous but intermittent upwelling, inducing high phytoplankton growth rates. However, export production (estimated from benthic foraminiferal accumulation rates) was low, possibly due to high regeneration rates in a deep thermocline. The upwelled waters may have been an analog to Subantarctic Mode Waters, carrying nutrients into the eastern Atlantic and western Indian Oceans as the result of the initiation of a deep-reaching Antarctic Circumpolar Current, active Agulhas Leakage, and vigorous vertical mixing in the Southern Oceans.

Late Pleistocene stratigraphy of IODP Site U1396 and compiled chronology offshore of south and south west Montserrat, Lesser Antilles

Marine sediments around volcanic islands contain an archive of volcaniclastic deposits, which can be used to reconstruct the volcanic history of an area. Such records hold many advantages over often incomplete terrestrial data sets. This includes the potential for precise and continuous dating of intervening sediment packages, which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores. Here we discuss a marine record of eruptive and mass-wasting events spanning ∼250 ka offshore of Montserrat, using new data from IODP Expedition 340, as well as previously collected cores. By using a combination of high-resolution oxygen isotope stratigraphy, AMS radiocarbon dating, biostratigraphy of foraminifera and calcareous nannofossils, and clast componentry, we identify five major events at Soufriere Hills volcano since 250 ka. Lateral correlations of these events across sediment cores collected offshore of the south and south west of Montserrat have improved our understanding of the timing, extent and associations between events in this area. Correlations reveal that powerful and potentially erosive density-currents traveled at least 33 km offshore and demonstrate that marine deposits, produced by eruption-fed and mass-wasting events on volcanic islands, are heterogeneous in their spatial distribution. Thus, multiple drilling/coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands. This multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during Expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands.

An early Miocene Atlantic-wide foraminiferal/palaeoceanographic event

Abstract In the lower Miocene deep-sea record of the Atlantic Ocean, a foraminiferal assemblage dominated by small, smooth, thin-walled bolivinids occurs at several sites, in different regions, from ∼20-17 Ma. No modern deep-sea analogues are known, but such assemblages are present in modern environments in the upper bathyal zone within the oxygen minimum zone. We interpret this as reflecting lowered oxygen conditions in the deep Atlantic Ocean, perhaps resulting from a period of sluggish bottom water circulation in the early Miocene ocean. This was a period of major change in global δ 13 C equivalent to the beginning of the Monterey Carbon Excursion.