Description: The internal microstructure of a ferromanganese nodule (#2392, from 154°37.52′W, 9°37.56′N, at water depth 5194 m) was examined in detail on polished sections, and radiometrically dated (230Thex/232Th) along a high-resolution (0.1 mm) depth profile (0–1.3 mm), spanning approximately 271 ka. The fabric shows typical stromatolithic structure and exhibits four orders of basic cyclic growth pattern, namely laminae bands, laminae zones, laminae groups and laminae pairs having average thicknesses of 402–454, 185–206, 58–67 and 15–18 μm, respectively. A profile from the depth of 200–2661 μm was selected to obtain the geochemical series using line-scanning electron microprobe analyses, which provide a record of paleoceanographic oscillations during the growth period. Power spectral analysis of the geochemical series for Al, Mn, Fe and Fe/Mn from the depth of 200–1220 μm, where no obvious discontinuity could be observed, display conspicuous cyclicities. The cycles of laminae zones, laminae groups and laminae pairs are reflected in the spectral patterns as well. The significant spectral peaks are located at 186, 108, 66 μm. Together with the cycle of laminae bands, the ratios of these cyclicities are close to those of Milankovitch orbital cycles. Through tuning to orbital cycles, a net growth rate of 4.5 mm/Ma is derived for the profile, which is in perfect agreement with the growth rate of 4.6 mm/Ma determined by 230Thex/232Th dating. Therefore, the rhythmic growth of ferromanganese nodules appears definitely associated with Milankovitch cycles, and the growth cyclicities may offer a new tool for estimating growth rates of ferromanganese nodules and paleoenvironmental reconstruction at substage resolution when supported by radiometric dating.

Description: Focused gas and fluid flow of cold seep ecosystems is often characterized by carbonate precipitation processes fueled by hydrocarbon-rich fluids and microbial activity. In various geological settings the seabed leakage is methane dominated and accompanied by the formation of long-lasting hard substrates and open channels. These fluid pathways are connecting deeper levels of the sediment column with the bottom water, bridging the diffusive processes at the sediment/water-interface. Understanding and quantifying feedback mechanisms between hydrocarbon-sources, ocean chemistry, and climate requires detailed data about the dynamics of seafloor methane emanation throughout geological time. Authigenic carbonates from these ecosystems represent in many cases unique archives of marine methane emanation by their geobiological, geochemical, mineralogical, and structural inventory. Precise and high resolution geochronology of these archives provides new insights into the rate and duration of precipitation processes, the related microbial activity and a base for the reconstruction of paleoactivity of natural seepage. The actual data set of our compilatory study is spanning a wide range in space and time. It covers different geological circum-Pacific settings (South China Sea, Costa Rica & Nicaragua, Chile, New Zealand), including more than 200 thousand years old archives (Hydrate Ridge, off Oregon) and recent methane-related carbonates from Black Sea and Mediterranean Sea. Special emphasis is actually given to new insights into growth structures, emplacement processes, mineralogy and high resolution geochemistry of mud mound and escarpment related carbonates from the Central American Forearc as well as to new findings on seep systems off Chile (cruise Sonne 210) applying a ROV-operated diamond chain saw sampling system. In a rather complex case study carbonate drill cores decipher the late stage evolution of mound growth and related methane enriched fluid emanation during the last 70 000 years off Costa Rica. A broad range in &13C from -22 to -36‰ (mounds) and -43 to -56‰ (escarpment) is covered, reflecting different hydrocarbon sources and/or varying fluid/seawater-ratios. Whereas the &18O signatures indicate a systematic variation between 3.8 - 5.3‰ (mounds) and 4.2 - 5.1‰ (escarpment) in close correlation with their ages (U-Th geochronology [1]) and the record of seawater evolution. Combining high resolution observations of growth structures (fluorescence microscopy) and analyses of Cl-S-C distribution pattern (electron microprobe) decipher multiple phases of carbonate precipitation separated by micrometer scaled layers of residual organic matter (e.g. 50 alternations on 3 mm). The latter are interpreted to be attached onto crystal surfaces during phases of rather stagnant or low fluid flow, respectively. On long time scales, the circum-Pacific data set indicates sea level decrease as an important enhancement factor for focused fluid flow via increasing pore water buoyancy, destabilization of gas hydrates and related fluxes from underlying free gas deposits upon hydraulic pressure release. Data from tectonically highly active settings imply structural changes as major control on initiation and position of cold seeps and their activation on short time scales [2, 3]. References: [1] Hammerich et al. (2007) Terra Nostra. [2] Kutterolf et al. (2008) Geology, doi: 10.1130/G24806A [3] Liebetrau et al. (2010) MG, doi:10.1016/j.margeo.2010.01.003.〈/p〉

Description: Cold seep ecosystems are often characterized by carbonate precipitation processes fueled by methane-rich fluids and microbial activity. Understanding and quantifying feedback mechanisms between methane sources, ocean chemistry, and climate requires detailed data about the dynamics of seafloor methane emanation throughout geological time. Carbonates from these ecosystems provide unique archives of marine methane emanation by their geobiological, geochemical, mineralogical, and structural inventory. Precise and high resolution geochronology of these archives provides new insights into the rate and duration of precipitation processes and the related microbial activity. In this compilatory study large carbonate samples from very different cold seep settings were investigated for the time scales of their formation and their specific precipitation environment. Beside new insights into growth structures, emplacement processes and initial approaches on high resolution geochemistry and biomarker analyses [1], special emphasis was given to the geochronological identification of paleo-seep-activity phases. The actual data set is spanning a wide range in space and time. It covers circum-Pacific settings (South China Sea, Costa Rica & Nicaragua, New Zealand), including more than 200 thousand years old archives ( Hydrate Ridge, off Oregon), and almost recent methane-related carbonates from Black Sea and Mediterranean Sea. On long time scales, the data indicates sea level decrease as an important enhancement factor for focused methane flux via destabilization of underlying gas hydrates upon hydraulic pressure release. Data from tectonically highly active settings imply structural changes as major control on initiation and position of cold seeps and their activation on short time scales [2, 3]. [1] Leefmann et al. (2008) BG. [2] Kutterolf et al. (2008) Geology, doi: 10.1130/G24806A [3] Liebetrau et al. (2010) MG, doi:10.1016/j.margeo.2010.01.003

Description: In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico.