Planktonic Oxygen Research Articles

Glacial‐interglacial dynamics of the eastern equatorial Pacific cold tongue‐Intertropical Convergence Zone system reconstructed from oxygen isotope records

We use planktonic oxygen isotope (δ18O) records spanning the last 30,000 years (kyr) to constrain the magnitude and spatial pattern of glacial cooling in the upwelling environment of the eastern equatorial Pacific (EEP). Fourteen new downcore δ18O records were obtained from surface‐dwelling planktonic foraminifera Globigerinoides sacculifer and Globigerinoides ruber in eight cores from the upwelling tongue of the EEP. All sites have sedimentation rates exceeding 5 cm/kyr and, with one exception, lie above the modern depth of the foraminiferal lysocline. Sites directly underlying the cool band of upwelling immediately south of the equator record mean late Holocene (LH)‐Last Glacial Maximum (LGM) δ18O amplitudes ranging between 1.0 and 1.3‰. We estimate that mean sea surface temperatures (SST) in this region during the LGM were on average 1.5 ± 0.5°C lower than the LH. Larger δ18O amplitudes are observed in sites north of the equator, indicating a spatial pattern of reduced meridional SST gradient across the equator during the LGM. This result is supported by comparison of Mg/Ca SST reconstructions from two sites straddling the equator. We interpret the reduction of this gradient during the LGM as evidence for a less intense cold tongue‐Intertropical Convergence Zone (ITCZ) frontal system, a more southerly position of the ITCZ, and weaker southeast equatorial trades in the EEP.

Climate change at the 4.2 ka BP termination of the Indus valley civilization and Holocene south Asian monsoon variability

Planktonic oxygen isotope ratios off the Indus delta reveal climate changes with a multi‐centennial pacing during the last 6 ka, with the most prominent change recorded at 4.2 ka BP. Opposing isotopic trends across the northern Arabian Sea surface at that time indicate a reduction in Indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south Asia. The 4.2 ka event is coherent with the termination of urban Harappan civilization in the Indus valley. Thus, drought may have initiated southeastward habitat tracking within the Harappan cultural domain. The late Holocene drought cycles following the 4.2 ka BP event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14C production rates. This suggests that solar variability is one fundamental cause behind Holocene rainfall changes over south Asia.

The Ka'ena Highstand of O'ahu, Hawai'i: Further Evidence of Antarctic Ice Collapse during the Middle Pleistocene

The Ka'ena Highstand of O'ahu, Hawai'i:Further Evidence of Antarctic Ice Collapse during the Middle Pleistocene1 Paul J. Hearty Abstract Marine isotope stage (MIS) 11 may well represent one of the most significant interglacial highstand events of the past million years. Ocean volume changes charted from coastal exposures imply partial or complete melting of some of the world's major ice caps during a middle Pleistocene interglacial. The coastal geology of both Bermuda and the Bahamas yields evidence of an MIS 11 highstand 20 m higher than present. Further support for this catastrophic episode in sea-level history is revealed in subtidal and intertidal deposits at +28±2 m in O'ahu, Hawai'i. The stratigraphy, petrology, and uplift history of the Hawaiian deposits strongly suggest a correlation with MIS 11, and a compilation of amino acid racemization, uranium/thorium (alpha and mass spectrometry), and electron spin resonance ages shows a scatter between 300 and 550 kyr. When corrected for uplift, the Ka'ena Highstand succession at Wai'anae Health Center (OWH1) reveals a "stepping up" of sea level through the interglaciation, similar to that described in the Bahamas. Previous studies on O'ahu attributed all 28 m elevation of the Ka'ena Highstand to uplift since 0.5 Ma, but now it appears that only 8 m of that was caused by uplift, and the remaining 20 m by eustatic sea-level rise. These findings from O'ahu strengthen evidence for the complete disintegration of the Greenland and West Antarctic ice sheets and partial melting of the East Antarctic ice sheet during the middle Pleistocene. If the instability of polar ice sheets can be linked to prolonged warm interglaciations as the data suggest, then existing conservative predictions for the magnitude of sea-level change by future "greenhouse" warming are seriously underestimated. A key to evaluating the potential threat of abrupt sea-level rise may be in understanding the behavior of ice sheets during past very warm and/or prolonged interglaciations. Of particular interest in assessing the Holocene climate, because of similar insolation values at that time, is marine isotopic stage (MIS) 11, which the deep-sea oxygen isotope records of ODP Site 677 (Shackleton et al. 1990) and SPECMAP (Imbrie et al. 1984) suggest occurred between 423 and 362 kyr (Figure 1) and lasted for 60 kyr. The deep-sea oceanographic evidence is compelling, yet remains at best an indirect or circumstantial measure of sea-level change during MIS 11. Other than direct studies at or beneath the ice sheet (Sherer et al. 1998), ocean volume changes in response to ice melting are best measured along stable shorelines or at locations where the tectonic element is well understood. Direct dating of MIS 11 tectonically elevated deposits near Rome, Italy, indicate that the interglacial highstand persisted for 15-37 kyr (Karner et al. 1999) and ended around 400 kyr ago (Karner and Renne 1998). A sequence of coral reefs and shoreline deposits of middle Pleistocene age between modern sea level and +28±2 m are well exposed along the west and northwest coasts between Ka'ena Point and Wai'anae town (Figure 2) and at lower [End Page 65] elevations (e.g., Black Point) elsewhere on O'ahu. Click for larger view View full resolution Figure 1. Planktonic oxygen isotope data from ODP Site 677 for the past million years (Shackleton et al. 1990). MIS 11 stands out as the isotopically lightest interglaciation over this interval of the Quaternary. Reviews concerning the sea-level, oceanographic, and ice history of MIS 11 are available (Burckle 1993, Oppenheimer 1998, Poore et al. 1999). On the North Slope of Alaska, Kaufman and Brigham-Grette (1993) and Brigham-Grette (1999) identified a +22 to +23 m middle Pleistoceneage shoreline and interpreted it to be associated with the Wainwrightian transgression. Similarly, in southern Britain, Bowen (1999) estimated an uplift-corrected sea-level highstand of +23 m and correlated this with MIS 11 on the basis of amino acid racemization (AAR) age estimates. Outcrops in Bermuda and the Bahamas reveal strong evidence of an important interglacial highstand with a rise of sea level to approximately +20 m during the middle Pleistocene. This...

Mud diapirs observed in two piston cores from the landward slope of the northern Ryukyu Trench, northwestern Pacific Ocean

Two piston cores were collected from the cones of two mud volcanoes identified by a side-scan sonar survey east of Tanega-shima Island on the landward slope of the northern Ryukyu Trench to clarify the internal nature of the mud diapir. The upper part (Unit I) of both cores consists of normal hemipelagic sediments, whereas the lower portions (Unit II) of the cores are composed of mud-supported breccia. Distinct decrease in porosity from Unit II to Unit I indicates that the pore fluid pressure for mud diapirism was changed after expulsion on the sea floor due to the extruding conditions associated with dewatering and degassing. The Late Eocene to Pleistocene planktonic foraminifera found in the clasts, and calcareous nannofossils from muddy matrix in Unit II suggest that the various-aged surrounding rocks of the diapiric vents were torn off and taken into the mud diapir via hydraulic fracturing under intrusive conditions. Based on the planktonic oxygen isotope stratigraphy, AMS 14C measurements and tephrochronology, Unit I represents continuous deposition of hemipelagic mud after ∼38 and ∼17 ka to the two cores, respectively, and marks the time cessation of diapiric activity in these mud volcanoes. The findings of this study reveal the detailed behavior of mud diapirs from first stage (intruding) to final stage (extruding) in diapirism. Although these mud volcanoes are located near sites, the activities show a wide age spectrum. Therefore, it is possible that there is a characteristic relationship between the mud diapir development and the tectonic setting of this region.

Link between the North and South Atlantic during the Heinrich events of the last glacial period

High resolution benthic oxygen isotope records combined with radiocarbon datings, from cores retrieved in the North, Equatorial, and South Atlantic are used to establish a reliable cronostratigraphy for the last 60 ky. This common temporal framework enables us to study the timing of the sub-Milankovitch climate variability in the entire surface Atlantic during this period, as reflected in planktonic oxygen isotope records. Variations in sea surface temperatures in the Equatorial and South Atlantic reveal two warm periods during the mid-stage 3 which are correlated to the warming observed in the North Atlantic after Heinrich events (HL) 5 and 4. However, the records show that the warming started about 1500 y earlier in the South Atlantic. A zonally averaged ocean circulation model simulates a similar north-south thermal antiphasing between the latitudes of our coring sites, when pertubated by a freshwater flux anomaly. We infer that the observed phase relationship between the northern and the southern Atlantic is related to periods of reduced NADW production in the North Atlantic, such as during HL5 and HL4.

Temporal variability of micro- and nanoplankton in the German Bight in relation to hydrographic structure and nutrient changes

Sources of variability include not only succession of plankton populations within the seasonal cycle but also changes in the hydrographic structure of the nearby stratified water masses of the outer German Bight. The expected eVect of increasing inorganic nutrient concentrations (eutrophication) on long-term changes of phytoplankton stocks could not be determined definitively, because of the overriding eVect of hydrographic changes. Indications for eutrophication eVects ‐ frequent plankton blooms and oxygen depletion in the bottom water due to accumulated organic particles ‐ are restricted to the outer German Bight. Diatoms and flagellates exhibit diVerent seasonal cycles, with particularly large inter-annual fluctuations in the summer dinoflagellate stocks. Nanoplankton populations appear to have increased sharply at the end of the 1970s. As other plankton and abiotic factors have changed in the southern North Sea at the same time, possible larger-scale events must be considered as well as regional causes. ? 1998 International Council for the Exploration of the Sea

Bacteria, Dissolved Organics and Oxygen Consumption in Salinity Stratified Chesapeake Bay, an Anoxia Paradigm

Chesapeake Bay is a bacterially dominated ecosystem driven, at least under summer conditions, by high levels of labile dissolved organics. Bacterioplankton are exceptionally abundant (20 × 109 cells liter−1) and productive (7 × 109 cells liter−1 d−1), and their biomass can equal or exceed 60% of phytoplankton biomass. In the salinity stratified Chesapeake Bay bacterioplankton account for 60–100% of planktonic oxygen consumption, potentially driving the Bay to anoxia in days to weeks. Sulfide, released from sediments by sulfate reducing bacteria, chemically consumes oxygen at rates up to 9 mg O2 liter−1 d−1 maintaining the oxygen deficit. The organic matter driving this oxygen demand in the summer season is functionally dissolved. Dissolved organics, measured as biochemical oxygen demand, account for about 60% of microbially labile organics throughout the water column and 80% (sometimes 100%) in the subpycnoclinal water. Field studies suggest that reduced oyster stocks in Chesapeake Bay may be a major factor in the shift to this bacterially dominated trophic structure

Overall planktonic oxygen and carbon dioxide metabolisms: the problem of reconciling observations and calculations of photosynthetic quotients

Overall planktonic oxygen and carbon dioxide metabolisms: the problem of reconciling observations and calculations of photosynthetic quotients

Benthic Foraminiferal Response to Glacial/Interglacial Episodes, Deep Gulf of Mexico: ABSTRACT

Q-mode factor analysis of 55 surface samples and 190 species from water depths beyond 1,500 m in the Gulf of Mexico defined an 8 assemblage model which accounted for 91% of the original information contained in the data matrix. Distributions of these 8 assemblages are related to variations in sediment terrestrial organic carbon content, calcium carbonate content, and water depth. No relationship could be defined for the 8 assemblages and overlying water temperature, salinity, dissolved O2 concentration, or median grain diameter of the sediment. Species counts on 58 samples at ~10 cm intervals from the USNS Kane core K-129 were completed in order to examine the response of benthic foraminiferal populations to glacial/interglacial episodes in the Gulf. The core is from a site at 3,108 m water depth in the southwestern Gulf (20°56.7^primeN, 95°05.7^primeW). It was chosen because it had been previously zoned on the basis of planktonic foraminifers and it spans a relatively long interval of time (~165,000 years). A 4 factor model of the core, produced by direct factor analysis, accounted for 93% of the original information of the 58 sample by 118 species matrix. An Eponides turgidus assemblage was found to dominate high calcium carbonate dissolution intervals through core K-129, whereas an Epistominella decorata assemblage dominates intervals of low carbonate dissolution. A Bulimina translucens assemblage shows highest factor loadings when surface water temperatures were elevated and continental glacial volumes were reduced. A Cibicidoides wuellerstorfi assemblage, which shows highest factor loadings during glacials, demonstrates a highly significant inverse relationship to the development of the Bulimina translucens assemblage. The results of this study suggest that the deep-water benthic foraminifers of the Gulf of Mexico have responded to climatic events of global and regional significance through the late Quaternary. Global climatic effects may be inferred from demonstrated relationships between the foraminiferal assemblages and the planktonic oxygen isotope record, and dissolution history of core K-129. A regional effect is suggested by the relationship defined for the Cibicidoides wuellerstorfi assemblage and the distribution of terrestrial organic carbon of the Gulf through time. End_of_Article - Last_Page 563------------

Origin, Depositional History, and Correlation of Miocene Diatomites Around North Pacific Margin: ABSTRACT

Distinctive Miocene diatomites and genetically related porcellanites form a remarkably widespread lithofacies within bathyal marine sequences around the margin of the North Pacific Ocean from Mexico to Korea. The Monterey Shale of California and the Onnagawa Formation of Japan typify these deposits. Planktonic correlations, radiometric dates, and oxygen isotope records indicate that the initial accumulation of the diatomites commenced in the early middle Miocene (15 Ma), coincident with initiation of a major climatic event marked by massive ice accumulation in Antarctica, steepening of the latitudinal temperature gradient, and increasingly vigorous surface circulation and primary productivity. Microfaunal and sedimentologic evidence demonstrates that the diatomites were c mmonly deposited in subsiding marginal basins characterized by sills intersecting intensified oxygen minima allowing preservation of laminated muds. Moreover, the diatomites occur within strikingly similar stratigraphic successions that suggest closely parallel tectonic, sedimentologic, and oceanographic development of Neogene marginal basins in this region. Typically, four major events can be recognized in these sequences: (1) deposition of Oligocene-lower Miocene volcanic rocks and continental and/or neritic marine clastics followed by (2) rapid margin subsidence, development of silled basins, and deposition of middle and upper Miocene diatomaceous sediments in essentially empty depocenters, (3) climatically induced elevation of the carbonate compensation depth and increasing influx of terrigenous debris in latest Miocene-earliest Pliocene time producing a carbonate-poor mudstone facies, and (4) introduction of rapidly deposited wedges of coarse terrigenous clastics during Pliocene-Pleistocene time that cap the underlying diatomites. This widespread and correlative stratigraphic pattern appears to be the combined product of (1) a major middle Cenozoic readjustment of Pacific plate margins resulting in synchronous development of Neogene marginal basins around the North Pacific rim and (2) coincident acceleration of diatom productivity in this region in response to severe deterioration of Neogene climate. End_of_Article - Last_Page 940------------