Sea-floor weathering

To extrapolate the sea-floor weathering flux Thydro into the past requires a model of the water-rock reaction. There are two important possibilities, depending on whether the reactable cations are more or less abundant than CO2. In equation (11) we implicitly assume that CO2 is quantitatively removed from the sea water. In effect we presume fast reactions with superabundant cations. Walker (1985) and Fran9ois Walker (1992) assumed that CO2 is removed in linear proportion to its concentration. This differs from our picture only in the value of -4hydro in either model it is the delivery of CO2-rich waters to the oceanic crust by hydrothermal circulation that limits uptake, so that CO2 is quantitatively depleted in the circulating sea water until available CaO, MgO and FeO in the rock are exhausted. 

Fig. 5. Global surface temperature and PCO2 histories for three high heat flow models with different values of a, the CO2 dependence of the sea-floor weathering rate. The models assume a current mid-ocean ridge flux Pridge = 2 X 10 moles a . Models with higher values of a require higher amounts of CO2 to obtain the same amount of CO2 consumption.

Fig. 10. Parameter choices that converge to modern atmospheric and continental CO2 inventories for models in which weathering rates increase abruptly upon the advent of roots. These use either p = 0.3 or p = 0.5 in equation (8), and reduce the silicate weathering constant (in equation (8)) before the advent of roots to 33% of what it is now. These values are consistent with Berner (1994, 1997)]. These models imply that the ancient atmosphere was fairly CO2 rich, and so sea-floor weathering needs to have been relatively inefficient or too much CO2 would have been captured by the mantle.

Figure 13. Plot of Li isotopic composition vs. inverse concentration for sea floor altered (weathered) mid-Atlantic ridge basalts (Chan et al. 1992). Solid line is the regression of the data (R = 0.927), the dashed line shows the predicted relation of a pure mixture of seawater with unaltered basalt (Teng et al. 2004), underscoring that the altered basalts incorporated Li into secondary mineral with a fractionation factor a -0.981.

The geological environments for these assemblages are those of weather ing, deep-sea floor sediments and continental shelf sediments, or shallow burial of these materials as sedimentary or tuffaceous rocks. 

Some of the lMd material (either illite or mixed-layer illite-montmorillonite) presumably formed authigenically on the sea bottom or on land from the weathering of K-feldspars however, much of it was formed after burial. Studies of Tertiary, Cretaceous, and Pennsylvanian thick shale sections (Weaver, 1961b) indicate that little lMd illite was formed at the time of deposition. These shales and many others contain an abundance of expanded 2 1 dioctahedral clays with a lMd structure, some of which is detrital and some of which formed by the alteration of volcanic material on the sea floor. With burial the percentage of contracted 10A layers systematically increases. 

Uplift returns carbon from the moderately and deeply buried sediments to the land. Both carbonate rocks and fossil organic carbon are exposed to weathering processes. For every mole of organic carbon buried on the sea floor and not respired-decayed by processes like ... 

Polynesians combined what they knew about the weather, winds, and currents to investigate the Pacific Ocean, while the Phoenicians, Greeks, and Arabs explored the Mediterranean Sea. The early Greeks in general and Herodotus (484-428 B.c.) in particular believed that the world was round. Heroditus performed studies of the Mediterranean, which helped sailors of his time. He was able to take depth measurements of the sea floor by using the fathom as a unit of measure, which was the length of a man s outstretched arms. Today the fathom has been standardized to measure 6 ft (1.8 m) in length. 

The expected vessel s movements were relevant to deformation of the superstructure however, displacements associated with sliding of the hull on the seabed could not be a-priori excluded, especially during extreme weather and marine conditions. Thus, a set of markers was placed on the sea floor in contact with the hull, periodically controlled by divers of the Italian Coast Guard and State Police. 

Figure 3 Marine organisms produce calcite at 4 times the rate at which the ingredients for this mineral are supplied to the sea by continental weathering and planetary outgassing. A transition zone separates the mid-depth ocean floor where calcite is largely preserved from the abyssal ocean floor where calcite is largely dissolved.

Calcium carbonate is currently more actively studied in aqueous solution. Earth s oceans are saturated with Ca2+ ions, especially near the surface. The calcium is delivered to the oceans mostly by rivers and streams. (As rocks weather, calcium ions are released and attracted to water molecules.) Because there is such a high concentration of calcium ions in the oceans, it easily precipitates out as calcium carbonate or, less commonly, as calcium sulfate. The precipitate contributes to the formation of coral reefs in warm lagoons and shallow tropical seas. Inland salt seas and lakes, such as the Dead Sea in Israel, the Great Salt Lake in Utah, and Mono Lake in eastern California, often have whitish limestone deposits around their shores. (All of these bodies of water are saltier than the oceans.) Softer deposits tend to be called chalk. In locations where spring water bubbles up through the floor of a salty lake, mounds of limestone, called tufas,... 

The leached ions are replenished by atmospheric fallout of sea spray entrapped in rain and by the formation of new igneous rock at the ocean floor. Assuming that the Na and Ca concentrations in the ocean are constant, them ocean residence time equals the replenishment rate of soils. Ocean residence time, in turn, is equal to the concentration divided by the rate of input from the world s rivers. The Na residence time in the oceans is about 100 million years the Ca residence time is about 1 million years K and Mg are intermediate. Comparing the residence times of Ca in soils and oceans shows that 200 to 400 m of soil are weathered of their Ca content during the 1 million years residence time of Ca in the oceans. 

The origin of authigenic minerals on the ocean floor has been extensively discussed in the past with emphasis on two major processes precipitation from solutions originating from submarine eruptions, and slow precipitation from sea water of dissolved elements, originating from weathering of continental rocks. It is concluded that in several marine authigenic mineral systems these processes overlap. ... 

Rock weathering is increased by the presence of bacteria, which release carbonates and allow them to be washed to the ocean. There they are incorporated into countless tiny lime shells by small sea creatures. When the creatures die, these shells rain down on the ocean floor. 

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