Kunio Kaiho

Benthic foraminiferal dissolved-oxygen index and dissolved-oxygen levels in the modern ocean

Changes in oxygen concentrations at the sediment-water interface play a major role in controlling benthic foraminiferal assemblages and morphologic characteristics; such changes are reflected in size, wall thickness, porosity, and also taxa (genera and species) of foraminifera present. These morphologic and taxonomic differences have been quantified as a dissolved-oxygen index. This paper demonstrates that the foraminiferal oxygen index derived from bathyal and abyssal Holocene faunas correlates well with the dissolved-oxygen levels in overlying waters. This index is then used for paleoenvironmental interpretations.

Effect of organic carbon flux and dissolved oxygen on the benthic foraminiferal oxygen index (BFOI)

Variations in oceanic primary productivity, flux of organic carbon to the sediments, and dissolved-oxygen levels in the water column are thought to be important in the control of benthic foraminiferal test size, wall thickness, morphology, and species composition of assemblages by many foraminiferal paleontologists. Aspects of these processes should be reflected by the benthic foraminiferal oxygen index (BFOI) based on these foraminiferal characteristics. However, analyses indicate that the BFOI correlates most strongly with dissolved-oxygen levels in overlying water (R2=0.81), weakly with oceanic primary productivity (R2=0.55), and weakly with organic carbon flux to the sediments (R2=0.51). Although both dissolved oxygen and organic carbon flux are main controlling factors for benthic foraminiferal assemblages, the BFOI is a useful indicator extracted from benthic foraminiferal assemblages for estimating the condition of dissolved oxygen in Cretaceous and Cenozoic oceans.

Latest Paleocene benthic foraminiferal extinction and environmental changes at Tawanui, New Zealand

A major extinction of intermediate-water (500–1000 m) benthic foraminiferal species coincided with a major decrease in δ13C (2.8‰) of terrestrial organic matter (n-C29 alkane) and δ34S (20‰) of whole rock sulfide in a continuous siltstone sequence in the Tawanui Section (46°S paleolatitude) along the Akitio River, southeastern North Island, New Zealand, in the middle part of the uppermost Paleocene nannofossil zone (CP8). The benthic extinction (25% of species) occurred over ∼3 kyr at ∼55.5 Ma. Increases in kaolinite/illite and kaolinite/smectite ratios and in terrestrial organic carbon percentages started ∼3 kyr before the major benthic extinctions, lasted over ∼40 kyr, and probably reflect warmer climate and increased rainfall. The productivity of planktonic foraminifera and calcareous nannoplankton decreased ∼3 kyr prior to the major extinctions and recovered at the time of benthic extinctions. These events that started ∼3 kyr before the extinction can be best explained by warming, increased rainfall, reduced salinity of surface waters, and increased influence of warm saline deep water (WSDW). Benthic foraminiferal oxygen indices indicate a strong decrease in dissolved oxygen levels within the intermediate water from low oxic (1.5–3.0 mL/L O2) to suboxic (0.3–1.5 mL/L O2) conditions coinciding with the benthic extinctions. Increases in total organic carbon (TOC) and in the hydrocarbon-generating potential of kerogen (measured as the hydrogen index (HI)) agree with the interpretation of decreased dissolved oxygen levels of the intermediate water. The lowest oxygen conditions lasted ∼40 kyr and coincided with a decrease in calcareous benthic foraminiferal productivity, highest TOC levels, and lowest δ13C of terrestrial organic carbon. Dominant formation of WSDW or sluggish intermediate-water circulation caused by warming and high rainfall in high-latitude areas most likely led to the ∼3-kyr time lag between events on land and in surface waters preceeding the extinction and the development of dysaerobia in the sea, coinciding with the major benthic extinction and decrease in δ13C and δ34S in New Zealand. Global warming of deep and intermediate waters may have caused decomposition of methane hydrate in sediments, resulting in a strongly decreased δ13C of marine carbonates, promoting dysaerobia in the ocean, and warming global climate by increased methane concentrations in the atmosphere. Upwelling of WSDW, occurring soon after it became dominant in high-latitude areas, is likely responsible for the recovery of normal salinity and the concomitant recovery of planktonic foraminifera and calcareous nannoplankton productivity in high-latitude surface waters. Minor benthic foraminiferal extinctions (9% of species) occurred ∼40 kyr after the major extinctions, lasted ≤ ∼6 kyr, and coincided with the initiation of environmental recovery.

Global changes of Paleogene aerobic/anaerobic benthic foraminifera and deep-sea circulation

Kaiho, K., 1991. Cenozoic global changes of aerobic/anaerobic benthic foraminifera and deep-sea circulation. Palaeogeogr., Palaeoclimalol., Palaeoecol., 83: 65-85. Morphologies of calcareous benthic foraminifera found in poorly oxygenated deposits differ from those present in highly oxygenated deposits. On the basis of this observation, l infer that benthic foraminiferal test morphology can be used to extrapolate relative amounts of dissolved oxygen in Paleogene deep-sea bottom water. Calcareous benthic foraminifera from DSDP samples of Cenozoic age from the world oceans and New Zealand Paleogene samples are classified into three categories: aerobic, anaerobic and intermediate forms. The ratio of aerobic versus aerobic plus anaerobic forms is useful for estimates of global changes in the oxygen content of deep oceanic waters. These data indicate that low oxygen deep-sea conditions developed in the world oceans in early Eocene and late Oligocene times. These low-oxygen events occurred coincidentally with episodes of oceanic warming and sluggish deep water circulation. This coincidence suggests that the main cause of the low-oxygen events is low velocity deep-sea circulation in those time intervals.

End-Cenomanian benthic foraminiferal extinctions and oceanic dysoxic events in the northwestern Pacific Ocean

Abstract The second-largest Cretaceous extinction event coincided with an oceanic anoxic event at the Cenomanian/Turonian (C/T) boundary. This high resolution analysis of upper bathyal benthic foraminifera in central Hokkaido, Japan indicates that the extinction event occurred in the latest Cenomanian over a period of ∼0.2 m.y. (93.6–93.4 Ma). Analysis of the dissolved oxygen index based on calcareous benthic foraminiferal wall thickness, size, and morphology shows that low oxygen, 0.1–1.5 ml/l conditions developed during a period of ∼0.15 m.y. before the C/T boundary and ∼1.5 m.y. after. The minimum oxygen condition (0.1–1 ml/l) prevailed twice during this interval. Similarly, benthic foraminiferal extinctions occurred twice in concert with initiation of the two minimum oxygen levels. Coincidence of the two extinction horizons with developemt of low oxygen conditions suggests that the main cause of the extinctions was formation of dysoxic intermediate-water. This report suggests that the global C/T boundary extinction event occurred in a stepwise fashion during the latest Cenomanian 0.2–0.5 m.y. due to development of widespread oceanic anoxic-dysoxic conditions within intermediate water masses.

Planktonic and benthic foraminiferal extinction events during the last 100 m.y.

Abstract Five major planktonic and benthic foraminiferal extinction events occurred during the past 100 m.y.: at the Cenomanian/Turonian ( C T) boundary, at the Cretaceous/Tertiary ( K T) boundary, in the latest Paleocene, in the late Eocene, and in the early middle Miocene. To clarify the character of the five events, I estimated (1) paleo-waterdepths and paleoenvironments where foraminifera were affected, (2) the duration of each extinction event, and (3) evaluated possible causes for each extinction event, on the basis of recent literature. These extinction events can be classified into three fundamentally different types: (1) the mass extinction event at the K T boundary was abrupt and concentrated among surface dwelling species, (2) extinctions at the C T boundary and during the latest Paleocene were abrupt and mainly affected intermediate- and deep-water taxa over short periods, (3) the late Eocene and early middle Miocene events affected surface- and deep-water species, and involved stepwise or gradual extinctions over a long period (∼ 4 m.y.). I attributed the extinction events to: (1) a bolide impact for the K T event, (2) a decrease in dissolved oxygen in intermediate- and deep-waters due to climatic warming for the C T and latest Paleocene events, and (3) long-term global cooling for the late Eocene and early middle Miocene events. The following relationships exist between the causes and types of extinction events: (1) large extraterrestrial impacts strongly affected surface-dwelling organisms in short-term extinction events, (2) intermediate- and deep-water low oxygen conditions induced by climatic warming have severely affected intermediate- and deep-water organisms over medium- and short-terms (

Spike of pyrosynthetic polycyclic aromatic hydrocarbons associated with an abrupt decrease in δ13C of a terrestrial biomarker at the Cretaceous-Tertiary boundary at Caravaca, Spain

The first vertical high-resolution record of polycyclic aromatic hydrocarbons (PAHs) of pyrosynthetic origin and the corresponding δ 13 C profile of a terrestrial biomarker across the Cretaceous-Tertiary (K-T) boundary at Caravaca, Spain, reveals the following. In comparison with adjacent Cretaceous marlstones, the first thin horizon (0 to +0.5 cm; 0 = the K-T boundary) of the boundary-clay layer is (1) enriched as much as 112 to 154 fold in typical pyrosynthetic PAHs such as coronene, benzo(g,h,i)perylene, and benzo(e)pyrene and (2) shows an abrupt δ 13 C decrease of 1.4‰–1.8‰ in terrestrial higher plant-derived n-C 29 alkane. The spike of pyrosynthetic PAHs associated with an abrupt decrease in δ 13 C value of a terrestrial biomarker is interpreted to reflect the prevalence of extensive fires with subsequent δ 13 C decrease in atmospheric CO 2 . It is estimated that the geologically instantaneous combustion of ∼18%–24% of the terrestrial above-ground biomass would be necessary to account for the measured negative isotopic shift at the K-T boundary, on the basis of carbon mass balance between terrestrial above-ground biomass and atmosphere.

Oceanic primary productivity and dissolved oxygen levels at the Cretaceous/Tertiary Boundary: Their decrease, subsequent warming, and recovery

Thirty-six different geochemical and foraminiferal analyses were conducted on samples collected at closely spaced intervals across the Cretaceous/Tertiary (K/T) boundary exposed at Caravaca, Spain. A rapid reduction in the gradient between δ13C values in fine fraction carbonate and benthic foraminiferal calcite and a decrease in the abundance of phosphorus (a proxy for organic carbon) and calcium were recorded in sediments 0–0.5 cm above the K/T boundary. These trends imply that an abrupt mass mortality occurred among pelagic organisms, leading to a significant reduction in the flux of organic carbon to the seafloor. In addition, variations in sulfur isotope ratios, the hydrocarbon-generating potential of kerogen (measured as the hydrogen index), and foraminiferal indices of dissolved oxygen level all imply that a rapid decrease in dissolved oxygen was coincident with the δ13C event. Evidence of the low oxygen event has also been recognized in Japan and New Zealand, suggesting that intermediate water oxygen minima were widely developed during earliest Danian time. A threefold increase in the kaolinite/illite ratio and a 1.2‰ decrease in δ18O (carbonate fine fraction) were recorded in the basal 0.1–2 cm of Danian age sediments. These trends suggest that atmospheric warming and an increase in surface water temperature occurred 0–3 kyr after the δ13C event. Recovery in the difference between δ13C values in the carbonate fine fraction and in benthic foraminiferal calcite as well as increases in phosphorus and calcium contents occur at the base of planktonic foraminiferal Zone Pla, implying that an increase in primary productivity commenced some 13 kyr after the K/T boundary. Tables A1-A3 are available on diskette or via Anonymous FTP from kosmos.agu.org directory APENO (Username = anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009 or by phone at 800-966-2481;

A low extinction rate of intermediate-water benthic foraminifera at the Cretaceous/Tertiary boundary

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Catastrophic extinction of planktonic foraminifera at the Cretaceous-Tertiary boundary evidenced by stable isotopes and foraminiferal abundance at Caravaca, Spain

The contrast between the high extinction rates of calcareous marine plankton and low extinction rates of deep-water calcareous benthic foraminifera across the Cretaceous/Tertiary (K/T) boundary is now recognized. However, the water depth at which the vertical change in extinction rates of marine organisms occurred at the K/T boundary is not yet known because of the lack of data on the extinction rates of intermediate-water organisms. The extinction rate of intermediate-water benthic foraminiferal species from Kawaruppu, Hokkaido, Japan, was determined by distinguishing locally extinct species. A distinctly low extinction rate (10%) for calcareous benthic foraminiferal fauna from the upper part of the intermediate-water is documented for the first time. This rate is almost equal to the extinction rate of deep-water benthic foraminifera (∼10%) and is much smaller than the planktic foraminiferal extinction rate (81%). The extinction rate curve for calcareous foraminifera in a vertical water column indicates that a rapid change from a high extinction rate (∼80%) to a low extinction rate (∼10%) took place at a water depth of about 150 m. This depth of change is equivalent to the boundary depth between the surface and intermediate waters and between the euphotic and aphotic zones. Furthermore, this coincidence is consistent with the hypothesis that acid rain and a darkness-induced breakdown of the marine food-chain, produced by a cometary or asteroidal impact at the K/T boundary, were the main causes of the mass extinction of calcareous planktic organisms in surface waters.