Robert V. Demicco

Secular variation in seawater chemistry and the origin of calcium chloride basinal brines

CaCl 2 basinal brines, which are present in most Phanerozoic sedimentary basins, inherited their chemistries and salinities from evaporated paleoseawaters when the world oceans were Ca rich and SO 4 poor (CaCl 2 seas). CaCl 2 seas coincided with periods of rapid seafloor spreading, high influxes of mid-ocean-ridge brines rich in CaCl 2 , and elevated sea levels, conditions that favored accumulation of marine CaCl 2 brines in marginal and interior continental basins. Typical basinal brines in Silurian-Devonian formations of the interior Illinois basin, United States, show the same compositional trends as those of progressively evaporated CaCl 2 -rich Silurian seawater. Chemical deviations can be accounted for quantitatively by brine-rock reactions during burial (dolomitization, dolomite and K-feldspar cement). This explanation for the origin of CaCl 2 basinal brines contrasts with others that assume constancy of seawater chemistry and involve more complex brine-rock interactions.

Atmospheric pCO2 since 60 Ma from records of seawater pH, calcium, and primary carbonate mineralogy

A 60 m.y. record of atmospheric p CO2 has been refined from knowledge of (1) secular changes in the major ion composition of seawater (particularly Ca and Mg) and (2) oscillations in the mineralogy of primary oceanic carbonate sediments. Both factors have had a significant impact on the chemistry of the ocean carbonate buffer system. Calculated atmospheric p CO2 oscillated between values of 100–300 ppm and to maxima of 1200–2500 ppm from 60 to 40 Ma and varied between 100 and 300 ppm from 25 Ma to the present. The refined p CO2 values are significantly lower than previous estimates made from seawater pH data where total dissolved inorganic carbon was assumed constant and more in line with modeling and stomatal index estimations of atmospheric p CO2 for the Tertiary.

Model of seawater composition for the Phanerozoic

We present an inverse model of Phanerozoic seawater com- position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites. The model considers step- wise alteration of seawater composition via: (1) variable input of river water, (2) variable rates of alteration of seawater through reactions at mid-ocean ridges, and (3) variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general. The model achieves agreement with paleoseawater fluid inclusion data for Na 1 ,C a 2 1 ,S O 4 2 2 , and K 1 , particularly when variable runoff is considered. Variable rates of basalt- seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater, particularly the observed, near-constant concentration of K 1 through time.

Wavy and lenticular-bedded carbonate ribbon rocks of the Upper Cambrian Conococheague Limestone, central Appalachians

ABSTRACT Thinly interbedded limestones and dolostones are common in Paleozoic and Proterozoic carbonate sequences. Such rocks, called ribbon rocks, are characteristic of the 750-m-thick Upper Cambrian Conococheague limestone, a platform deposit exposed in western Maryland. Limestones of Conococheague ribbon rocks occur as continuous wavy thin beds to isolated centimeter-scale pods; commonly have flat bottoms and ripple-form tops; typically are internally current- or wave-ripple cross-laminated; and are composed of peloidal grainstones. Dolostones interbedded with these limestones are typically continuous on the outcrop scale; drape underlying limestone ripple-forms; are commonly mudcracked; and are composed of 10-40 B. dolomite rhombohedrons in a carbonate-clay matrix. Some ribbon rocks are di rupted by filled tubes, soft sediment deformation features, and abrupt lateral changes in rock type. Ribbon rocks within meter-scale cycles of the Conococheague have gradational contacts with cross-stratified grainstones below and grade into mud-cracked laminites above. These observations suggest that the limestone-dolostone alternations of ribbon rocks reflect original carbonate sand-mud alternations. This interpretation is strongly amplified by comparing Conococheague ribbon rocks to examples of lenticular and wavy bedding from mixed siliciclastic sand and mud flats from the modern North Sea. Conococheague ribbon rocks are interpreted as ancient carbonate examples of lenticular and wavy bedding deposited on mixed carbonate sand and mudflats in a shallow subtidal to intertidal setting. This is consistent with an overall interpretation of Conococheague cycles as the regressive deposits of carbonate tidal flats.

Modeling seafloor-spreading rates through time

A steady-state model of crust production and destruction for the past 180 m.y. was proposed by B. Parsons and advocated by D. Rowley. Such a model has serious implications for models of secular variations in, e.g., global sea level, global climate, and seawater chemistry. This paper presents an analysis of the steady-state model and then offers alternative extensions of that model that allow for non–steady-state production of ocean crust with time. Results suggest that the observed linear decrease in area versus age of ocean floor does not force a steady-state view of seafloor spreading.

Evaluating seawater chemistry from fluid inclusions in halite: examples from modern marine and nonmarine environments

Fluid inclusions from marine halites have long been studied to determine the chemical composition of ancient seawater. Chemical analyses of the major ions in fluid inclusions in halites from the solar saltwork of Great Inagua Island, Bahamas, and from the supratidal sabkha, Baja California, Mexico, show that modern marine halites faithfully record the chemical signature of seawater. The major ions in Great Inagua and Baja California fluid inclusions display distinctive linear trends when plotted against one another (ie., Na+, K+, and SO42− vs. Mg2+ and Cl−), which track the evaporation path of seawater as it evolved during the crystallization of halite. These evaporation paths defined for the major ions by fluid inclusions in halite overlap findings of computer simulations of the evaporation of modern seawater by the Harvie, Moller, and Weare (HMW) computer program. The close match between the HMW seawater evaporation paths and the Great Inagua fluid inclusion data is not surprising considering the carefully controlled inflow, evaporation, and discharge of seawater at the Great Inagua saltwork. The major ion chemistry of fluid inclusions from the Baja California halites matches the HMW seawater evaporation paths in most respects, but one Baja fluid inclusion has lower concentrations of Mg2+ than evaporated seawater. Nonmarine inflows and syndepositional recycling of preexisting salts in the Baja California supratidal setting were not large enough to override the chemical signature of evaporating seawater as the primary control on the Baja fluid inclusion compositions. Fluid inclusions in halites from the nonmarine Qaidam Basin, Qinghai Province, western China, have a distinctly different major ion chemical signature than does “global” seawater. The fluid inclusion chemistries from the Qaidam Basin halites do not lie on the evaporation pathways defined by modern seawater and can clearly be differentiated from fluid inclusions containing evaporated seawater. If fluid inclusions in halites from modern natural settings contain unmistakable samples of evaporated seawater, then evaluation of the chemistry of ancient seawater by chemical analysis of fluid inclusions in ancient marine halites by means of the same approach should be valid.

CYCOPATH 2D—a two-dimensional forward model of cyclic sedimentation on carbonate platforms

Abstract This paper documents CYCOPATH 2D, a FORTRAN computer code for 2-dimensional modeling of cyclostratigraphic pathways within carbonate platform deposits. The code allows the users options over four suites of variables: subsidence; sediment production; sea level fluctuations; and sediment transport. CYCOPATH 2D presents an animated display of the changes in the sediment surface of a carbonate platform during deposition. At the end of a run, a summary stratigraphic cross-section is drawn on the screen. Data on the sea level history of the run, details of the depositional status of the platform for every time step, and details for plotting individual stratigraphic columns are saved to output files for each run. CYCOPATH 2D incorporates lateral transport of sediment onto shoreline wedges and can produce autocycles and allocycles. Given the same sets of input, CYCOPATH 2D substantially reproduces the synthetic stratigraphies produced by the “Dr. Sediment” code of Dunn. CYCOPATH 2D is the only generally available model of carbonate deposition that can test hypotheses that include the possibility of autocyclic generation of stacked peritidal shallowing upwards successions.

Eocene atmospheric CO2 from the nahcolite proxy

Estimates of the atmospheric concentration of CO 2 , [CO 2 ] atm , for the “hothouse” climate of the early Eocene climatic optimum (EECO) vary for different proxies. Extensive beds of the mineral nahcolite (NaHCO 3 ) in evaporite deposits of the Green River Formation, Piceance Creek Basin, Colorado, USA, previously established [CO 2 ] atm for the EECO to be >1125 ppm by volume (ppm). Here, we present experimental data that revise the sodium carbonate mineral equilibria as a function of [CO 2 ] and temperature. Co-precipitation of nahcolite and halite (NaCl) now establishes a well-constrained lower [CO 2 ] atm limit of 680 ppm for the EECO. Paleotemperature estimates from leaf fossils and fluid inclusions in halite suggest an upper limit for [CO 2 ] atm in the EECO from the nahcolite proxy of ∼1260 ppm. These data support a causal connection between elevated [CO 2 ] atm and early Eocene global warmth, but at significantly lower [CO 2 ] atm than previously thought, which suggests that ancient climates on Earth may have been more sensitive to a doubling of [CO 2 ] atm than is currently assumed.

The Carbonate Factory Revisited: A Reexamination of Sediment Production Functions Used to Model Deposition on Carbonate Platforms

There are two widely used models of sediment production on carbonate shelves: (1) production varying with depth, and (2) production as a function of depth and distance from the platform margin. Here we recalculate carbonate production on Great Bahama Bank west and northwest of Andros Island using Broecker and Takahashis (1966) pioneering geochemical study. In the calculations, residence time and depth are used to calculate yearly carbonate production based on equilibrium thermodynamics of CaCO3 loss from water over the banks. Whereas Broecker and Takahashi assumed a uniform depth of 4.5 m in their work, here we use realistic depths and residence times interpolated over the bank. Our results suggest that sediment production is on the order of 4-5 kg/m2/y on the bank margins and that production rapidly drops off into the platform, approaching zero in the shallowest, innermost parts of the platform. Thus, for a large platform such as Great Bahama Bank with long residence times, the shallowest areas of the platform may become sinks, not sources, of sediment.

Stratigraphic simulations using fuzzy logic to model sediment dispersal

Abstract The purpose of this paper is to report on our preliminary two- and three-dimensional stratigraphic simulations that use fuzzy logic to model sediment production, sediment erosion, sediment transport and sediment deposition. Fuzzy logic offers a robust, easily adaptable, and computationally efficient alternative to the traditional numerical solution of complex, coupled differential equations commonly used to model sediment dispersal in stratigraphic models. Fuzzy logic is based on the concept of fuzzy sets, and, since the 1980s, fuzzy logic has been successfully applied in virtually all areas of engineering and computer sciences, as well as in areas of decision making, optimization, management, and operations research. Fuzzy logic is also rapidly being assimilated into the sciences and, since it is capable of utilizing both “hard” data and “soft” qualitative statements, fuzzy logic naturally lends itself to applications in the Earth Sciences. Here we first compare two-dimensional simulations of reef growth: one based on step-wise solution of a partial differential equation and one in which an elementary fuzzy logic system is employed. The two simulations produce identical results. We then present three fully three-dimensional models: (1) a simulation of the last 200,000 of sedimentation in Death Valley, CA; (2) a simulation of sedimentation on the Great Bahama Banks west of Andros Island during the latest 10,000 years of sea level rise; and (3) a hypothetical delta and floodplain under varying regimes of sea level change. The results of the first two models match surface and subsurface data from Death Valley and the Great Bahama Bank to a remarkable degree even though the models are in preliminary stages. Moreover, the hypothetical deltaic simulations also produce remarkably complex and realistic cross-sections. Thus, our preliminary modeling suggest that the utility of fuzzy logic in stratigraphic simulations may be profound.