Geochemistry of dark-light rhythm sediments of the Japan Sea

Ocean Drilling Program Legs 127 and 128 in the Japan Sea have revealed the existence of numerous dark-light rhythms of remarkable consistency in sediments of late Miocene, latest Pliocene, and especially Pleistocene age. Light-colored units within these rhythms are massive or bioturbated, consist of diatomaceous clays, silty clays, or nannofossil-rich clays, and are generally poor in organic matter. Dark-colored units are homogeneous, laminated, or thinly bedded and include substantial amounts of biogenic material such as well-preserved diatoms, planktonic foraminifers, calcareous nannofossils, and organic matter (maximum 7.4 wt%). The dark-light rhythms show a similar geometrical pattern on three different scales: First-order rhythms consist of a cluster dominated by dark-colored units followed by a cluster dominated by light-colored units (3-5 m). Spectral analysis of a gray-value time series suggests that the frequencies of the first-order rhythms in sediments of latest Pliocene and Pleistocene age correlate to the obliquity and the eccentricity cycles. The second-order dark-light rhythms include a light and a dark-colored unit (10-160 cm). They were formed in time spans of several hundred to several ten thousand years, with variance centering around 10,500 yr. This frequency may correspond to half the precessional cycle. Third-order rhythms appear as laminated or thinly bedded dark-light couplets (2-15 mm) within the dark-colored units of the second-order rhythms and may represent annual frequencies. In interpreting the rhythms, we have to take into account that (1) the occurrence of the first- and second-order rhythms is not necessarily restricted to glacial or interglacial periods as is shown by preliminary stable-isotope analysis and comparison with the published d18O record; (2) they appear to be Milankovitch-controlled; and (3) a significant number of the rhythms are sharply bounded. The origin of the dark-light rhythms is probably related to variations in monsoonal activity in the Japan Sea, which show annual frequencies, but also operates in phase with the orbital cycles.

Magnetic properties of light and dark sediment layers from the Japan Sea

Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K, Kfd, ARM, SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers, testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations, with glacial stages characterized by a lower magnetic mineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35-0.40 Ma ago.

Effects of feeding and light intensity on the response of the coral Porites rus to ocean acidification

Recently, it has been suggested that there are conditions under which some coral species appear to be resistant to the effects of ocean acidification. To test if such resistance can be explained by environmental factors such as light and food availability, the present study investigated the effect of 3 feeding regimes crossed with 2 light levels on the response of the coral Porites rus to 2 levels of pCO2 at 28 °C. After 1, 2, and 3 weeks of incubation under experimental conditions, none of the factors-including pCO2-significantly affected area-normalized calcification and biomass-normalized calcification. Biomass also was unaffected during the first 2 weeks, but after 3 weeks, corals that were fed had more biomass per unit area than starved corals. These results suggest that P. rus is resistant to short-term exposure to high pCO2, regardless of food availability and light intensity. P. rus might therefore represent a model system for exploring the genetic basis of tolerance to OA.

Mud, sand, heavy and light minerals composition in lower Cretaceous at DSDP Hole 93-603B

During the drilling of Hole 603B on Deep Sea Drilling Project Leg 93, an unexpected series of sand-, silt-, and claystone turbidites was encountered from Cores 603B-45 through -76 (1224-1512 m sub-bottom depth). Complete and truncated Bouma sequences were observed, some indicating deposition by debris flows. Sand emplacement culminated with the deposition of a 30-m-thick, unconsolidated sand unit (Cores 603B-48 through -45). The purpose of this preliminary study is to determine the nature of the heavy mineral suites of this sediment in order to make tentative correlations with onshore equivalents. The heavy mineralogy of Lower Cretaceous North American mid-Atlantic coastal plain sediment has been extensively studied. This sediment is classified as the Potomac Group, which has a varied heavy mineral suite in its lower part (Patuxent Formation), and a limited suite in its upper part (Patapsco Formation). The results of this study reveal a similar trend in the heavy mineral suites of sediment in Hole 603B. Hauterivian through lower Barremian sediment has a heavy mineral suite that is dominated by zircon, apatite, and garnet, with minor amounts of staurolite and kyanite. Beginning in the mid-Barremian, a new source of sediment becomes dominant, one which supplies an epidote-rich heavy mineral suite. The results of the textural analyses show that average grain size of the light mineral fraction increases upsection, whereas sorting decreases. The epidote-rich source may have delivered sediment with a slightly coarser mean grain size. This sediment may represent a more direct continental input at times of maximum turbidite activity (mid-Barremian) and during deposition of the upper, unconsolidated sand unit.

Seawater carbonate chemistry and combined physiological effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101 during experiments, 2010

Recent studies on the diazotrophic cyanobacterium Trichodesmium erythraeum(IMS101) showed that increasing CO2 partial pressure (pCO2) enhances N2 fixation and growth. Significant uncertainties remain as to the degree of the sensitivity to pCO2, its modification by other environmental factors, and underlying processes causing these responses. To address these questions, we examined the responses ofTrichodesmium IMS101 grown under a matrix of low and high levels of pCO2 (150 and 900 µatm) and irradiance (50 and 200 µmol photons m-2 s-1). Growth rates as well as cellular carbon and nitrogen contents increased with increasing pCO2 and light levels in the cultures. The pCO2-dependent stimulation in organic carbon and nitrogen production was highest under low light. High pCO2 stimulated rates of N2fixation and prolonged the duration, while high light affected maximum rates only. Gross photosynthesis increased with light but did not change with pCO2. HCO3- was identified as the predominant carbon source taken up in all treatments. Inorganic carbon uptake increased with light, but only gross CO2 uptake was enhanced under high pCO2. A comparison between carbon fluxes in vivo and those derived from 13C fractionation indicates high internal carbon cycling, especially in the low-pCO2treatment under high light. Light-dependent oxygen uptake was only detected underlow pCO2 combined with high light or when low-light-acclimated cells were exposed to high light, indicating that the Mehler reaction functions also as a photoprotective mechanism in Trichodesmium. Our data confirm the pronounced pCO2 effect on N2fixation and growth in Trichodesmium and further show a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of carbon and nitrogen under elevated pCO2. These findings are supported by a complementarystudy looking at photosynthetic fluorescence parameters of photosystem II, photosynthetic unit stoichiometry (photosystem I:photosystem II), and pool sizes of key proteins in carbon and nitrogen acquisition.