Calcium carbonate preservation in the equatorial Pacific

The Pliocene-Pleistocene history of CaCO3 preservation in the central equatorial Pacific is reconstructed from a suite of deep-sea cores and is compared to fluctuations in global ice volume inferred from delta18O records. The results are highlighted by: (1) a strong covariation between CaCO3 preservation and ice volume over 104 to 106 year time scales; (2) a long-term increase in ice volume and CaCO3 preservation since 3.9 Ma demonstrated by a deepening of the lysocline and the carbonate critical depth; (3) a dramatic shift to greater CaCO3 preservation at 2.9 Ma; (4) distinctive ice-volume growth and CaCO3 preservation events at 2.4 Ma, which are associated with the significant intensification of northern hemisphere glaciation; (5) a mid-Pleistocene transition to 100-kyr cyclicity in both CaCO3 preservation and ice volume; and (6) a 600-kyr Brunhes dissolution cycle superimposed on the late Pleistocene glacial/interglacial 100-kyr cycles. CaCO3 preservation primarily reflects the carbonate chemistry of abyssal waters and is controlled by long-term (106 year) and short-term (104 to 105 year) biogeochemical cycling and by distinct paleoclimatic events. We attribute the long-term increase in CaCO3 preservation primarily to a fractionation of CaCO3 deposition from continental shelf to ocean basin, and secondarily to a gradual rise in the riverine and glaciofluvial flux of Ca++. On shorter time scales, the fluctuations in CaCO3 preservation slightly lag ice volume fluctuations and are attributed to climatically induced changes in the circulation and chemistry of Pacific deep water.

(Table 2) Relative contents of clay minerals in aeolian pelagic clays from the Pacific Ocean

For determining importance, composition, and history of aerosol material accumulation in formation of pelagic clays a study with use of light microscopy and scanning electron microscopy, X-ray diffractometry, and chemical methods has been carried out.

(Plate 3) Manganese nodules and deep-sea sediment core from Pacific Ocean

A synoptic review of the studies of well-known occurrences of palagonite tuffs is presented. Included are palagonite tuffs from Iceland, and pillow-lava palagonite complexes from Columbia River basalts and from the central Oregon coast. Additional petrologic and x-ray defraction data for selected samples are presented. Petrologic evidence shows that basaltic glass of aqueous tuffs and breccias consists of sideromelane, which is susceptible to palagonitization. It is shown that palagonitization is a selective alteration process, involving hydration, oxidation and zeolitization. Some of the manganese nodules dredged from the Pacific Ocean floor contain nucleus of palagonite-tuff breccias or of zeolite. A brief megascopic and microscopic description of nodules from the south Pacific, the Mendocino ridge and the 'Horizon' Nodule from the north Pacific is presented. Petrographic studies of palagonite-tuff breccias of manganese nodules and other palagonites suggest that migration and segregation of metallic elements occur during and subsequent to palagonitization. During the palagonitization of sideromelane, nearly 30 percent of sea water is absorbed. The hydration of sideromelane is also accompanied by oxidation of iron and other elements. These oxides may be released either in colloidal form or in true solution and tend to precipitate first from the unstable palagonite.

Grain size and mineral compositions of Pacific red clays

Hypotheses of origin of ocean deep red clays are under discussion. On an example of the Pacific Ocean grain size, mineralogy and chemical composition of clays are considered. It is shown that they formed from atmospheric dust and andesite pyroclastics. Accumulation of the clays occurred through deposition particle-by-particle and by pellet transport.

Barite accumulation rates throughout Pliocene-Pleistocene in Eastern Equatorial Pacific at ODP Site 138-849

Export production is an important component of the carbon cycle, modulating the climate system by transferring CO2 from the atmosphere to the deep ocean via the biological pump. Here we use barite accumulation rates to reconstruct export production in the eastern equatorial Pacific over the past 4.3 Ma. We find that export production fluctuated considerably on multiple time scales. Export production was on average higher (51 g C/m**2/yr) during the Pliocene than the Pleistocene (40 g C/m**2/yr), decreasing between 3 and 1 Ma (from more than 60 to 20 g C/m**2/yr) followed by an increase over the last million years. These trends likely reflect basin-scale changes in nutrient inventory and ocean circulation. Our record reveals decoupling between export production and temperatures on these long (million years) time scale. On orbital time scales, export production was generally higher during cold periods (glacial maxima) between 4.3 and 1.1 Ma. This could be due to stronger wind stress and higher upwelling rates during glacial periods. A shift in the timing of maximum export production to deglaciations is seen in the last ~1.1 million years. Results from this study suggest that, in the eastern equatorial Pacific, mechanisms that affect nutrient supply and/or ecosystem structure and in turn carbon export on orbital time scales differ from those operating on longer time scales and that processes linking export production and climate-modulated oceanic conditions changed about 1.1 million years ago. These observations should be accounted for in climate models to ensure better predictions of future climate change.

(Table 1, page 258) Chemical composition of 24 concentric layers inside manganese nodule DH9-9 from the Pacific Ocean

The minor-element composition of concentric layers within a single ferromanganese nodule from the eastern North Pacific exhibits strong correlations with Fe and Mn contents but appears to be independent of pronounced mineralogic variations. On the basis of these correlations, the elemental composition of individual layers apparently is controlled by the relative contribution of two sources: seawater, and interstitial water of associated sediment. In contrast, the mineralogy of the nodule, consisting of birnessite in the outer few layers and todorokite in the inner layers, is considered to be a function of nodule diagenesis.

Long-term biogenic particle fluxes in the Bering Sea and the central subarctic Pacific Ocean

Time-series sediment traps were deployed for five consecutive years in two distinctively different subarctic marine environments. The centrally located subarctic pelagic Station SA (49°N, 174°W; water depth 5406 m) was simultaneously studied along with the marginal sea Station AB (53.5°N, 177°W; water depth 3788 m) in the Aleutian Basin of the Bering Sea. A mooring system was tethered to the sea-floor with a PARFLUX type trap with 13 sample bottles, which was placed at 600 m above the sea-floor at each of the two stations. Sampling intervals were synchronized at the stations, and they were generally set for 20 days during highly productive seasons, spring through fall, and 56 days during winter months of low productivity. Total mass fluxes, which consisted of mainly biogenic phases, were significantly greater at the marginal sea Station AB than at the pelagic Station SA for the first four years and moderately greater for the last year of the observations. This reflects the generally recognized higher productivity in the Bering Sea. Temporal excursion patterns of the mass fluxes at the two stations generally were in parallel, implying that temporal changes in their biological productivity are strongly governed by a large-scale seasonal climatic variability over the region rather than local phenomena. The primary reason for the difference in total mass flux at the two stations stems mainly from varying contributions of siliceous and calcareous planktonic assemblages. A significantly higher opal contribution at Station AB than at Station SA was mainly due to diatoms. Diatom fluxes at the marginal sea station were about twice those observed at the pelagic station, resulting in a very high opal contribution at Station AB. In contrast to the opal fluxes, CaCO3 fluxes at Station AB were slightly lower than at Station SA. The ratios of Corg/Cinorg were usually significantly greater than one in both regions, suggesting that preferentially greater organic carbon from cytoplasm than skeletal inorganic carbon was exported from the surface layers. Such a process, known as the biological pump, leads to a carbon sink which effectively lowers p CO2 in the surface layers and then allows a net flux of atmospheric CO2 into the surface layer. The efficiency of the biological pump is greater in the Bering Sea than at the open-ocean station.

Stable isotope ratios of benthic and planktonic foraminifera from Miocene to Pliocene sediments of DSDP Hole 7-62A on the Eauripik Rise, Pacific Ocean (Table 1)

Oxygen and carbon isotope analyses show that the biserial forarniniferal genus Streptochilus, which was originally described from pelagic sediments on the Eauripik Rise and Ontong Java Plateau, lived deep in the upper water column within the oxygen minimum layer. The species of Streptochilus average from 4 to 19% of the foraminiferal assemblages in which benthic forms compose less than 1 or 2%. Specimens of Streptochilus are selectively dissolved when in contact with the bottom water mass. Their rapid evolutionary turnover of less than a few million years and their wide areal distribution in the equatorial Indo-Pacific are indicative of planktonic foraminifera. Aside from usefulness of the species of Streptochilus as stratigraphic indices, these Neogene biserial planktonic foraminifera are potential indices of paleoceanographic stratification.

Oligocene to early Miocene benthic foraminifera in the eastern Equatorial Pacific

We investigated Oligocene and early Miocene benthic foraminiferal faunas (> 105 µm in size) from Ocean Drilling Program (Leg 199) Site 1218 (4826 m water depth and ~3300 to ~4000 m paleo-water depth) and Site 1219 (5063 m water depth and ~4200 to ~4400 m paleo-water depth) to understand the response of abyssal benthic foraminifera to mid-Oligocene glacial events in the eastern Equatorial Pacific Ocean. Two principal factor assemblages were recognized. The Factor 1 assemblage (common Nuttallides umbonifer) is related to either an influx of the Southern Component Water (SCW), possibly carbonate undersaturated, or a decrease in seasonality of the food supply from the surface ocean. The Factor 2 assemblage is characterized by typical deep-sea taxa living under variable trophic conditions, possibly with a seasonal component in food supply. The occurrence of abyssal benthic foraminifera faunas during the mid-Oligocene depends on either the effect of SCW or the seasonality of food resources. The Factor 1 assemblage was most common near 76Ol-C11r, 73Ol-C10rn and 67Ol-C9n (ca. 30.2, 29.1 and 26.8 Ma respectively by Pälike et al. (2006, doi:10.1126/science.1133822)). This indicates that the effect of SCW increased or the seasonal input of food from the surface ocean to benthic environments was weakened close to these glacial events. In contrast, the huge export flux of small biogenic carbonate particles close to these glacial events might be responsible for carbonate-rich sediments buffering carbonate undersaturation. Changes in deep-water masses or the periodicity of food supply from the surface ocean and variation in surface carbonate production affected by orbital forcing had an impact on the mid-Oligocene faunas of abyssal benthic foraminifera around the intervals of glacial events in the eastern Equatorial Pacific Ocean. The Factor 1 assemblage decreased sharply at ? 30 Ma (29.8 Ma by Pälike et al. (2006), 30.0 Ma by CK95) and returned to dominance after ? 29 Ma (28.6 Ma by Pälike et al. (2006), 28.8 Ma by CK95). It is likely that the effect of SCW (possibly carbonate undersaturated) has intensified since the late Oligocene. The faunal transition of benthic foraminifera in the eastern Equatorial Pacific Ocean at ~29 Ma might be attributable to the influence of Northern Component Water (NCW) input to the Southern Ocean and the subsequent formation of SCW at about that time.

Late Cenozoic stable isotope record of benthic and planktonic foraminifera from the Pacific Ocean

Stable isotopic analyses of Middle Miocene to Quaternary foraminiferal calcite from east equatorial and central north Pacific DSDP cores have provided much new informatlon on the paleoceanography of the Pacific Neogene The history of delta18O change in planktonic foraminifera reflects the changing Isotopic composition and temperature of seawater at the time of test formation. Changes in the isotopic composition of benthonic foraminifera largely reflect changes m the volume of continental ice. Isotopic data from these cores indicates the following sequence of events related to continental glaciation (1) A permanent Antarctic ice sheet developed late in the Middle Miocene (about 13 to 11.5 m.y. ago) (2) The Late Miocene (about 11.5 to 5 m.y. ago) is marked by significant variation in delta18O of about 0.5? throughout, indicating instability of Antarctic ice cap size or bottom-water temperatures (3) The early Pliocene (5 to about 3 m.y. ago) was a time of relative stability in ice volume and bottom-water temperature (4) Growth of permanent Northern Hemisphere ice sheets is referred to have begun about 3 m.y. ago (5) The late Pliocene (3 to about 1.8 m.y. ago) is marked by one major glaciation or bottom-water cooling dated between about 2.1 to 2.3 m.y. (6) There is some evidence that the frequency of glacial-interglacial cycles increased at about 0.9 m.y. There is significant variation in delta13C at these sites but no geochemical interpretation is offered in this paper. The most outstanding feature of delta13C results is a permanent shift of about -0.8? found at about 6.5 m.y. in east equatorial and central north Pacific benthonic foraminifera. This benthonic carbon shift may form a useful marker in deep-sea cores recovering Late Miocene carbonates.

Pelagic phosphorus accumulation in the Pacific Ocean

As a limiting nutrient to marine life, phosphorus (P) is an effective tracer of today's marine productivity. The distribution of P in marine sediments likewise tracks the history of marine productivity because of its relative insolubility in seawater. CaCO3, biogenic opal, terrigenous sediment, and total P have been measured in cores from nine Pacific sites (Deep Sea Drilling Project (DSDP) 65, 66, 310, 77, 62, 572, 463, 586, and GPC-3) and one subantarctic (DSDP 266) site. These sites were specifically chosen to provide information on biota burial flux changes with time for sedimentary sinks that represent key oceanographic variables, i.e., rate of upwelling, water depth, and carbonate dissolution gradient. The accumulation rates of these components for the last 10 Ma were then calculated from determined core age versus depth plots, core bulk density, and porosity data. The accumulation of P weakly correlates with that of CaCO3, moderately with that of total sediment, and very strongly with carbonate-free accumulation. Two prominent peaks for all components occur at 2-3 Ma and 5-6 Ma, and record the chemical loading of dissolved CaCO3, SiO2, and P from glacially emergent continental shelves. These results indicate that continental shelf phosphorites form during interglacially high sea levels and correspond to low deep-sea P accumulation rates, whereas glacially lowered sea levels allow for shelf bypassing and greater deep-sea P accumulation rates.

(Table 1) Stable oxygen isotope ratios of benthic foraminifera from Pacific Ocean deep-sea sediments

The thermal structure of the Pacific Ocean between water depths of about 1 and 4.5 kilometers is estimated from the oxygen isotopic ratio of benthonic foraminifera from deep-drilled and piston cores of early Pliocene age (about 3 to 5 million years ago). The ratio of oxygen-18 to oxygen-16 in the early Pliocene at each site varies by an average of only ± 0.12 per mil (1 standard deviation). A plot of the oxygen isotopic ratio against modern bottom-water temperature is adequately fit by a line having a slope of - 0.26 per mil per degree Celsius (the equilibrium temperature dependence of calcite-water fractionation), suggesting that the temperature gradient of the Pacific Ocean during the early Pliocene was similar to that of today.

Annotated record of the detailed examination of Mn deposits from R/V Xiang Yang Hong 16 stations (1983) in the Pacific Ocean

Electron microprobe and X-ray diffraction data for north Pacific manganese nodules reveal that the transition metal distributions are controlled by the mineralogy. Microlayers rich in 10Å-manganates generally have high Mn/Fe ratios and positive correlations between Ni, Cu and Mn, and between Co and Fe. Microlayers rich in vernadite, on the other hand, show low Mn/Fe ratios, and Co, Ni and Cu all show positive correlations with Mn. The 10Å-manganates form mainly in porewaters with high Mn/Fe ratios. The Ni2+ and Cu2+ ions are post-depositionally incorporated into the interlayers of the manganates, whereas Co3+ is substituted for Fe3+ in ferric oxyhydroxides. In seawater with a low Mn/Fe ratio, on the other hand, the adsorption of positively charged ferric oxyhydroxides on negatively charged [MnO6] octahedral layers suppresses the growth of 10Å-manganates, enhancing the formation of vernadite. Positively charged hydroxides of Co3+, Ni2+ and Cu2+ are also adsorbed on the [MnO6] layers. These mechanisms of mineral formation and metal uptake are corroborated by data for other oceanic non-hydrothermal manganese nodules and crusts.