Corrosive Bottom Waters Research Articles

Middle Eocene–Early Pliocene Subantarctic planktic foraminiferal biostratigraphy of Site 1090, Agulhas Ridge

The analysis of planktic foraminiferal assemblages from Site 1090 (ODP Leg 177), located in the central part of the Subantarctic Zone south of South Africa, provided a geochronology of a 330-m-thick sequence spanning the Middle Eocene to Early Pliocene. A sequence of discrete bioevents enables the calibration of the Antarctic Paleogene (AP) Zonation with lower latitude biozonal schemes for the Middle–Late Eocene interval. In spite of the poor recovery of planktic foraminiferal assemblages, a correlation with the lower latitude standard planktic foraminiferal zonations has been attempted for the whole surveyed interval. Identified bioevents have been tentatively calibrated to the geomagnetic polarity time scale following the biochronology of Berggren et al. (1995). Besides planktic foraminiferal bioevents, the disappearance of the benthic foraminifera Nuttallides truempyi has been used to approximate the Middle/Late Eocene boundary. A hiatus of at least 11.7 Myr occurs between ∼78 and ∼71 m composite depth extending from the Early Miocene to the latest Miocene–Early Pliocene. Middle Eocene assemblages exhibit a temperate affinity, while the loss of several planktic foraminiferal species by late Middle to early Late Eocene time reflects cooling. During the Late Eocene–Oligocene intense dissolution caused impoverishment of planktic foraminiferal assemblages possibly following the emplacement of cold, corrosive bottom waters. Two warming peaks are, however, observed: the late Middle Eocene is marked by the invasion of the warmer water Acarinina spinuloinflata and Hantkenina alabamensis at 40.5 Ma, while the middle Late Eocene experienced the immigration of some globigerinathekids including Globigerinatheka luterbacheri and Globigerinatheka cf. semiinvoluta at 34.3 Ma. A more continuous record is observed for the Early Miocene and the Late Miocene–Early Pliocene where planktic foraminiferal assemblages show a distinct affinity with southern mid- to high-latitude faunas.

Characteristics of Seafloor Morphology and Ferromanganese Nodule Occurrence in the Korea Deep-sea Environmental Study (KODES) Area, NE Equatorial Pacific

Seafloor morphology and ferromanganese nodule occurrence were studied using a multibeam side scan sonar (SeaBeam, 2000) and a deep-sea camera system in the Korea Deep-sea Environmental Study (KODES) area, northeast equatorial Pacific. Seafloor morphology and nodule abundance are highly variable even in this small study area. The NNE-SSW oriented hills are parallel and about 100–200 m high. Valleys are very flat-floored, while hilltops are rugged with depressions of tens of meters. Cliffs to about 100 m bound the valleys and the hills. The study area can be classified into three types based both on nodule occurrence and seafloor morphology, mostly G- and B-types and some M-type. G-type is characterized by high nodule abundance, ubiquitous bioturbation, and flat seafloor morphology, while B-type is characterized by irregular-shaped nodules, variable nodule abundance, occurrence of giant nodules and sediment lumps, rugged bottom morphology with depressions, and white calcareous surface sediments. Medium nodule abundance and a generally flat seafloor characterize M-type. G-type occurs mostly in the valley regions, while B-type is on the hilltop areas. M-type is located between the hilltop and the valley. Tectonic movement of the Pacific plate resulted in the elongated abyssal hills and cliffs. The rugged morphology on hilltops resulted from erosion and redistribution of surface siliceous sediments on hilltops by bottom currents, outcropping of underlying calcareous sediments, and dissolution of the carbonate sediments by corrosive bottom water undersaturated with CaCO3. Sediment eroded from the hills, which is relatively young and organic-rich, is deposited in the valleys, and diagenetic metal supply to manganese nodules in the valley area is more active than on the hills. We suggest that tectonic movement ultimately constrains morphology, surface sediment facies, bottom currents and sediment redistribution, bioturbation, thickness of the sedimentary layer, and other conditions, which are all interrelated and control nodule occurrence. The best potential area for mining in the study area is the G-type valley zones with about 3–4 km width and NNW-SSE orientation.

Holocene paleoceanography in the southeast Venezuela Basin, Caribbean Sea

Abundance variations of benthic foraminifera in four box cores (3775–4447 m water depth) from the southeastern Venezuela Basin reflect the Holocene paleoclimatic history of the basin. There are no significant but some subtle paleoclimaticl changes during the Holocene. Overall, the following nine dominant calcareous benthic foraminifera species fluctuate down-core but display no obvious stratigraphic trends:Cibicides wuellerstorfi, Nuttallides decorata, Eponides polius, E. turgidus, E. tumidulus, Pullenia quinqueloba, P. bulloides, Melonis pompilioides, andOridorsalis tener. Primitive species of arenaceous foraminifera are abundant, while more complex forms are rare. Arenaceous foraminifera disappear at 16–18 cm depth in all four cores suggesting that below 16–18 cm the sediment is affected by diagenesis. Significant down-core dissolution of carbonate is seen in the deepest core 68-8 (4447 m). Except core 68-8, the late Holocene is characterized by a higher abundance of theGloborotalia menardii complex, a higher percentage of calcium carbonate, and a greater coarse fraction. Relatively large values for both calcium carbonate and coarse fraction percentages are caused by high productivity in warm surface waters. The late Holocene record of the foraminifera shows greater calcium carbonate dissolution, higher productivity (fertility), and more efficient water mixing during deposition than the early Holocene record. Corrosive bottom water masses are also indicated in the late Holocene by brown color and a high degree of broken planktonic foraminifera. The homogeneity of the southeastern Venezuela basin water mass is responsible for the small variations of S, H(S), and E.

Changes in Arctic Ocean paleoproductivity and hydrography during the last 145 kyr: The benthic foraminiferal record

The benthic foraminiferal record of two sediment cores at 900 and 2500 rn water depth in the marginal Arctic Ocean reflects fluctuations in paleoproductivity, ice sheet extent, and Atlantic water inflow. Highest paleoproductivity is observed in interglacial periods, and at the termination of interstadials to stadials within oxygen isotope substages (OIS) 6.3, 5.3, 5.1, 3.1, and Termination Ia. Stable and high paleoproductivity at the ice‐edge during terminations is indicated by the Melonis zaandami benthic foraminiferal association. Periods of glacial ice sheet advance or retreat from the core site are dominated by benthic foraminiferal associations related to seasonal organic carbon flux. Temperate saline Atlantic water entered the Arctic Ocean within OIS 6.3, 5.5 and the Holocene. Coincident paleoproductivity maxima are related to extended seasonal ice retreat. During glacial periods of stagnating Atlantic water advection unusual low paleoproductivity values are observed in the Arctic Ocean, indicating increasing ice coverage. Deep water was poorly ventilated during these periods and the deeper site became exposed to corrosive bottom waters.

Changing depth distribution of hiatuses during the Cenozoic

The differential effects of climate change, sea level, and water mass circulation on deposition/erosion of marine sediments can be constrained from the distribution of unconformities in the world's oceans. I identified temporal and depth patterns of hiatuses (“hiatus events”) from a large and chronologically well constrained stratigraphic database of deep‐sea sediments. The Paleogene is characterized by few, several million year long hiatuses. The most significant Cenozoic hiatus event spans most of the Paleocene. The Neogene is characterized by short, frequent hiatus events nearly synchronous in shallow and deep water sediments. Epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution. The Plio‐Pleistocene is characterized by a gradual decrease in the frequency of hiatuses. Future studies will focus on the regional significance of the hiatus events and their possible causes.

Pliocene - Pleistocene Benthic Foraminifera and Abyssal Circulation Changes at Arabian Sea DSDP Sites 220 and 223

Abstract Pliocene-Pleistocene benthic foraminifera were analysed from arabian Sea Deep Sea Drilling Project (DSDP) sites 220 and 223. At site 220 the most dominant taxon is Nuttallides umbonifera showing peak abundances in the Early Pliocene, Early and latest Pleistocene possibly representing intervals of highly carbonate-undersaturated bottom waters. The species diversity [H(S)] and equitability (E') trends show inverse relationship with N. umbonifera percentages. At relatively shallower site 223, the benthic assemblage is dominated by Cibicides wuellerstorfi, Oridorsalis umbonatus and Uvigerina hispido-costata through Plio-Pleistocene. N. umbonifera shows peak abundances only in the beginning of the Pliocene. This species shows an abrupt decrease to near extinction near the Plio-Pleistocene boundary and occurs very rarely and sporadically in the Pleistocene at site 223. C. wuellerstorfi and O. umbonatus show increased percentages in the earliest Pliocene, late Early Pliocene, and a short lived peakin the Early Pleistocene (samples 2-5), indicating presenceof relatively warm, oxygenated, and less corrosive bottom water at this site. The peaks of U.hispido-costata at site 223 are inferred to reflect intervals of high surface productivity due to intense upwelling off the Arabian coast.

Waxing (and Waning) lyrical on hiatuses: Eocene ‐ Quaternary Indian Ocean hiatuses as proxy indicators of water mass production

The temporal distributions of Cenozoic hiatuses in the Indian Ocean are reconsidered in the light of a new time framework developed by the Paleoceanographic Indian Ocean Synthesis. On the basis of the temporal distribution of hiatus terminations, eight 3 m.y. time slices were selected for plotting the paleogeographical distribution of hiatus terminations. These distributions define areas of potential hiatus development, which formed along flow paths of proto‐Antarctic Bottom Water (an analogue of modern Antarctic Bottom Water) and proto‐Antarctic Intermediate Water (an analogue of modern Antarctic Intermediate Water). Expansion and contraction of these areas suggested changes in the volume of these water masses, which modified the distibution patterns of erosive deepwater currents and corrosive bottom water. Consequently, the spatial and bathymetric distribution of hiatus formation was altered. Age‐depth analysis of hiatuses from sites within post‐late Eocene areas of potential hiatus development was used, in conjunction with models of the behavior of unconformity boundaries within fluctuating water masses, to construct waxing/waning curves for proto‐Antarctic Bottom Water and proto‐Antarctic Intermediate Water. These curves define volume changes in the production histories of these water masses which were related to the post‐late Eocene glacial history of Antarctica. Since the late Oligocene, the proto‐Antarctic Bottom Water waned during intervals of increased glacial intensity, whereas the proto‐Antarctic Intermediate Water waxed during these intervals. These relationships reversed during intervals of reduced glacial intensity, including intervals where there is no evidence for Antarctic glaciation. This conflicts with interpretations made by other workers. A comparison is made between the production histories of proto‐Antarctric Bottom Water and Northern Component Water (an analogue of North Atlantic Deep Water) based on ∂13C for the Miocene to Pliocene.

Evolution of the Norwegian Current and the Scandinavian Ice Sheets during the past 2.6 m.y.: evidence from ODP Leg 104 biogenic carbonate and terrigenous records

Records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2.6 m.y. of ODP Sites 643 and 644 in order to reconstruct surface and deep water regimes in the Norwegian Sea. Surface water regimes record long lasting moderate glacial conditions during the interval 2.6–1.0 Ma. Small intrusions of Atlantic water episodically penetrated into the Norwegian Sea forming a narrow tongue along the eastern margin, which is documented at Site 644. The polar front was most probably situated between the Site 644 and 643 locations on the outer Vøring Plateau during these time intervals. Deep water regimes reflect long-term persistent corrosive bottom waters, most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters. Deep water production in the Norwegian-Greenland Sea may have operated in a different way, e.g. brine formation during winter sea ice growth. Bottom waters were oxygenated throughout the entire period, and deep water was exchanged persistently with the North Atlantic. Increased glacial/interglacial environmental contrasts are documented, reflecting a strengthening of the Norwegian Current and intensified glaciations on the surrounding land masses during the interval 1.0–0.6 Ma. During this time a major shift in the mode of deep water production occurred. The onset of large amplitudes in glacial/interglacial environmental conditions with maximum contrasts in surface water regimes, different modes of deep water production, and intensified exchange with the North Atlantic marks the last 0.6 Ma. A broad development of the Norwegian Current is observed during peak interglacials, while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions.

Late Pleistocene‐Holocene benthic foraminiferal distribution in the southwestern Barents Sea: Paleoenvironmental implications

A benthic foraminiferal stratigraphy from the southwestern Barents Sea indicates that foraminifera were reworked and deposited in tills during the last glaciation. The deglaciation occurred in three main steps: (1) Presence of an Elphidium excavatum dominated assemblage > 13,300–12,000 B.P. (2), Nonion barleeanum dominated assemblage 12,000–10,000 B.P., and (3) establishment of a fauna similar to the modern one at 10,000 B.P. The transition from step 1 to step 2 indicates that the deglacial warming/incipient intrusion of Atlantic water was delayed in the southwestern Barents Sea compared with the western margin of the Norwegian shelf by approximately 1,000 years. Corrosive bottom water that formed during the last deglaciation causing carbonate dissolution may be due to poor ventilation or increased biogenic production accompanying the inferred oceanographic changes.

Late Quaternary carbonate sedimentation and paleo‐oceanography in the eastern Norwegian Sea

Four gravity cores from the eastern Norwegian Sea are studied. Absolute accumulation rates are quantified and variations in carbonate sedimentation and their implications for the paleo‐oceanographic history of the Norwegian Sea are described. In the eastern Norwegian Sea, interglacial, ice‐free conditions were developed during oxygen‐isotope stages 1 and 5e. Open water conditions were probably the norm during the summer season, also during glacial stages. Slightly elevated summer temperatures in periods during isotope stages 2 and 7 are demonstrated by increased contents of subpolar planktic foraminifera. The deep waters of the eastern Norwegian Sea have been well oxygenated during most of the last 250,000 years. Organic‐rich sediments and intensive carbonate dissolution in some parts of isotope stages 4 and 6 indicate corrosive bottom waters. A permanent ice cover and low saline surface waters, as found in the Arctic Ocean today, may have been developed in these periods. Well‐preserved foraminiferal assemblages from stage 2 show more oxygenated bottom waters and more effective bottom water renewal in this period than during stage 3.

A calcite dissolution pulse in the Norwegian-Greenland Sea during the last deglaciation

Carbonate preservation profiles in the Atlantic Ocean and the Norwegian-Greenland Sea seem to be out-of phase during the last deglaciation. A possibly time transgressive deglacial preservation spike in the N-Atlantic overlaps with a major calcite dissolution pulse in the Norwegian Sea. Contemporaneously major changes in surface and bottom water circulation of the Norwegian-Greenland Sea occurred. Isotopic and sedimentological evidence suggests that bottom water formation in the Norwegian-Greenland Sea was almost shut down during the last maximum glaciation and probably also during the first part of the last deglaciation. During that time corrosive bottom waters might have filled the Norwegian Sea deep sea basins causing carbonate dissolution at the sea floor. Subsequently, the reinitiation of deep water formation could have been coupled with increased resuspension of organic matter and/or reworking of older organic matter rising the p CO2 of bottom waters and contributing to carbonate dissolution at the sea floor. Additionally, large volumes of atmospheric CO2 stored before the Younger Dryas might have been pumped into the deep sea possibly by downwelling of surface waters and been neutralized against carbonate at the sea floor.

Eocene -Oligocene paleoceanography of the deep Bay of Biscay: Benthic foraminiferal evidence

A major change in benthic foraminiferal assemblages occurred in the deep Bay of Biscay (> 3 km water; DSDP Sites 119, and Site 400A) between early middle Eocene and earliest Oligocene. Predominant Eocene deep-sea taxa ( Nuttallides truempyi, Clinapertina spp., Abyssamina spp.) and associated rarer species became extinct in this interval. These extinctions were followed by an increase in abundance of bathymetrically wide-ranging and stratigraphically long-ranging taxa: Globocassidulina subglobosa, Oridorsalis spp., Gyroidinoides spp., and the Cibicidoides ungerianus plexus. The extinctions cannot be dated precisely from the stratigraphic record recovered to date in the Bay of Biscay; however, the replacement of the N. truempyi-dominated assemblage has been noted previously in the deep South Atlantic/Caribbean as occurring near the middle/late Eocene boundary. Other than the decrease in abundance and extinction of N. truempyi, no major abundance changes are noted within the Eocene at the shallower Site 401 (∼ 2 km water) in the Bay of Biscay. During the Oligocene, Nuttallides umbonifera replaced the Eocene species N. truempyi as the predominant deep-sea benthic foraminifera, reaching peak abundance in the middle Oligocene at Sites 119 and Site 400A. In the modern oceans, the abundance ot N. umbonifera is positively correlated with increased corrosiveness of bottom water, while at Site 119 the abundance of Nuttallides spp. is negatively correlated with δ 13C values in benthic foraminifera. As lower δ 13C values are often associated with older water masses, large numbers of Nuttallides spp. are thought to reflect older, and more corrosive bottom water. The faunal data and oxygen and carbon isotopic data are compared with a circulation model derived from North Atlantic seismic stratigraphic studies to show that old, warm, corrosive, and sluggish Eocene bottom water was replaced by younger, colder, less corrosive, more vigorously circulating bottom water of northern origin by the early Oligocene. Faunal and isotopic data suggest that bottom water became older and more corrosive again in the middle Oligocene, reflecting a reduction in circulation that can also be inferred from the seismic record in the nearby Rockall Plateau region.