Mass balance ocean composition

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

An important aspect in the preceding discussion is the need to separate the fluid and sediment components spatially and (as we will see) also temporally. Quantitative mass balance estimates (e g., McCulloch and Gamble 1991 Stolper and Newman 1994 Ayers 1998) often conclude that there is as much, or even more, Th and U in the bulk slab componenf (i.e., sediment plus fluid from the altered oceanic crust). However, if the sediment component added is in U-Th isotope equilibrium (or returns to this state prior to fluid addition see Section 5.3), then addition of only 0.02 ppm U in the fluid will result in significant U-excesss in the composite source (e.g., Condomines and Sigmarsson 1993 Turner et al. 1997). 

Figure 6. Summary of existing Mo isotope data from natural samples. Isotopic composition of ocean source is based on mass balance (see text). Data are presented as 5 Mo and S Mo relative to the Rochester JMC standard (5 Mo 2/3 x 5 Mo). References ( ) McManus et al. 2002 (t) Siebert et al. 2003 ( ) Barling et ah 2001 ( ) Arnold et al. 2004. Molybdenite values of Wieser and DeLaeter (2003) are omitted because of standard normalization problems (see text). Data obtained by different research groups using different standards are cross-calibrated by comparing seawater 5 values.

Using average MORE or the range of compositions of oceanic basalts (e.g., Hofmann, 1988 Chapter 3.13 and http //petdb.ldeo.columbia.edu Lehnert et al., 2000), the fluxes derived here can be applied to determine the average compositions of oceanic crust that is subducted and recycled into the mantle. These compositions thus influence the composition of subduction zone magmas (see Chapter 3.18) and bear on the chemical mass balance of the mantle. 

Tans (1980) laid out a useful approach to assess the isotopic mass balance of atmospheric CO2 as a function of the fluxes, isotopic compositions and isotopic fractionations involved in the transfer of CO2 between the atmosphere, the ocean and the biosphere. The simple formulation shown below involves some approximation that is justihed in light of the uncertainties in the system (Tans, 1980). Accordingly, the mean temporal change in atmospheric CO2 content (Cf) can be described by... 

Input is balanced by output in a steady-state system. The concentration of an element in seawater remains constant if it is added to the sea at the same rate that it is removed from the ocean water by sedimentation. Input into the oceans consists primarily of (1) dissolved and particulate matter carried by streams, (2) volcanic hot spring and basalt material introduced directly, and (3) atmospheric inputs. Often the latter two processes can be neglected in the mass balance. Output is primarily by sedimentation occasionally, emission into the atmosphere may have to be considered. Note that the system considered is a single box model of the sea, that is, an ocean of constant volume, constant temperature and pressure, and uniform composition. 

When air masses containing water vapor evaporated from the ocean surface are transported to colder regions via global wind patterns, there is a loss of water vapor because the vapor pressure of water decreases progressively with lower temperature. Because the condensate (rain) is approximately 9%o more enriched than the water vapor, by mass balance the water remaining in the cloud must become progressively more depleted in (and D). Thus, an ocean-derived cloud cooled from 20 to 10 °C would lose approximately half its initial vapor content (Fig. 5.10) and in the process decrease its isotopic composition from - 9 to - 17%o. The rain falling from the cloud at 10 °C would follow this depletion in and thus would have... 

Beyond the broad major-element constraints afforded by seismic imaging, the abundance of many trace elements in the mantle clearly records the extraction of core (Chapters 2.01 and 2.15) and continental crust (Chapter 2.03). Estimates of the bulk composition of continental cmst (Volume 3) show it to be tremendously enriched compared to any estimate of the bulk Earth in certain elements that are incompatible in the minerals that make up the mantle. Because the crust contains more than its share of these elements, there must be complementary regions in the mantle depleted of these elements—and there are. The most voluminous magmatic system on Earth, the mid-ocean ridges, almost invariably erupt basalts that are depleted in the elements that are enriched in the continental crust (Chapter 2.03). Many attempts have been made to calculate the amount of mantle depleted by continent formation, but the result depends on which group of elements is used and the assumed composition of both the crust and the depleted mantle. If one uses the more enriched estimates of bulk-continent composition, the less depleted estimates for average depleted mantle, and the most incompatible elements, then the mass-balance calculations allow the whole mantle to have been depleted by continent formation. If one uses elements that are not so severely enriched in the continental cmst, for example, samarium and neodymium, then smaller volumes of depleted mantle are required in order to satisfy simultaneously the abundance of these elements in the continental cmst and the quite significant fractionation of these elements in the depleted mantle as indicated by neodymium isotope systematics. 

Crust-mantle chemical mass-balance models offer important constraints on compositional variations in the mantle, but their constraints on the size of the various reservoirs involved depend critically on uncertainties in the estimates of the bulk composition of the continental crust, the degree of depletion of the complementary depleted mantle, and the existence of enriched reservoirs in Earth s interior, for example, possibly significant volumes of subducted oceanic crust. This last item was left out of the mass-balance models that suggested that the upper and lower mantle are chemically distinct. Chapter 2.03 makes it clear that much of the chemical and isotopic heterogeneity observed in oceanic volcanic rocks reflects various mixtures of depleted mantle with different types of recycled subducted crust. With this realization, and excepting the noble gas evidence for undegassed mantle, some of the characteristics of what was once labeled... 

Better constraints are required for the input parameters (particularly for the noble gases and nitrogen). Additional information on the volatile composition of both oceanic sediments and crustal basement is needed to improve estimates of mass balance at subduction zones. 

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