Lawrence A. Hardie

Secular variation in seawater chemistry: An explanation for the coupled secular variation in the mineralogies of marine limestones and potash evaporites over the past 600 m.y.

Secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites occur in phase on a 100–200 m.y. time scale such that periods of “aragonite seas” are synchronized with MgSO4 evaporites and periods of “calcite seas” with KCl evaporites. It is proposed that these coupled changes are the result of secular variation in seawater chemistry controlled primarily by fluctuations in the mid-ocean ridge hydrothermal brine flux, which in turn have been driven by fluctuations in the rate of ocean crust production. Quantitative predictions based on this hypothesis yield secular variation in limestone and potash evaporite mineralogies that closely match the observed variation over the past 600 m.y., providing strong support for the thesis that seawater chemistry, rather than remaining constant, has oscillated significantly over geologic time.

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.

Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming

The Wilkins Peak Member of the Green River Formation of Wyoming has been examined in outcrop with the object of reconstructing its depositional environment. Based on their assemblages of sedimentary structures, seven rock units are described, six of which define depositional subenvironments. These units are (1) flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil shale, (5) tronahalite, (6) siliciclastic sandstone, and (7) volcanic tuff. Their respective subenvironments are (1) rapid transgression of a shallow lake, (2) lake shore oscillating over a mud flat (slow transgression), (3) playa mud flats, (4) shallow lake with occasional desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) not specific. These subenvironment deposits are arranged in depositional cycles. We have observed four types of cycles involving flat-pebble conglomerate (A), oil shale (B), mudstone (C), lime sandstone (D), and also trona. These cycles are I: A-B-C, II: D-B-C, III: D-C, and IV: B-trona-C. Individual cycles have been correlated over distances of up to 24 km. The Wilkins Peak Member is thought to have been deposited in a playa-lake complex, which consisted of a shallow, central playa lake that was surrounded by vast, normally exposed mud flats fringed by alluvial fans. Evaporative concentration of bicarbonate-rich inflow waters led to saturation with respect to calcite, most of which must have been deposited as cement within alluvial fans. Evaporation continued in the capillary zone of the mud flats, precipitating calcite first, then magnesian calcite, and eventually protodolomite. The carbonates accumulated as a soft micritic mud at the fringes of the playa mud flats. During periods of desiccation, the muds were subject to cracking, and the mud-crack polygons contributed sand- and silt-size dolomitic micrite intraclasts that were transported to the central lake by the next storm. When the central lake was large, oil shale accumulated in it, with the organic matter derived from a flocculent ooze consisting of bottom-dwelling blue-green algae and fungi. During dry periods the lake shrank, and trona and halite precipitated in the central portions. An understanding of the Wilkins Peak sediments can be achieved only by considering as inseparable the hydrologic, sedimentary, geochemical, and biologic processes responsible for their formation.

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.

Low-magnesium calcite produced by coralline algae in seawater of Late Cretaceous composition

Shifts in the Mg/Ca ratio of seawater driven by changes in midocean ridge spreading rates have produced oscillations in the mineralogy of nonskeletal carbonate precipitates from seawater on time scales of 108 years. Since Cambrian time, skeletal mineralogies of anatomically simple organisms functioning as major reef builders or producers of shallow marine limestones have generally corresponded in mineral composition to nonskeletal precipitates. Here we report on experiments showing that the ambient Mg/Ca ratio actually governs the skeletal mineralogy of some simple organisms. In modern seas, coralline algae produce skeletons of high-Mg calcite (>4 mol % MgCO3). We grew three species of these algae in artificial seawaters having three different Mg/Ca ratios. All of the species incorporated amounts of Mg into their skeletons in proportion to the ambient Mg/Ca ratio, mimicking the pattern for nonskeletal precipitation. Thus, the algae calcified as if they were simply inducing precipitation from seawater through their consumption of CO2 for photosynthesis; presumably organic templates specify the calcite crystal structure of their skeletons. In artificial seawater with the low Mg/Ca ratio of Late Cretaceous seas, the algae in our experiments produced low-Mg calcite (<4 mol % MgCO3), the carbonate mineral formed by nonskeletal precipitation in those ancient seas. Our results suggest that many taxa that produce high-Mg calcite today produced low-Mg calcite in Late Cretaceous seas.

Secular variations in Precambrian seawater chemistry and the timing of Precambrian aragonite seas and calcite seas

An extension of the seawater secular-variation model that successfully predicted the observed timing of Phanerozoic MgSO 4 vs. KCl marine evaporites and aragonite seas vs. calcite seas has been applied to the prediction of the secular variations in the major ion chemistry of seawater and aragonite seas vs. calcite seas during the Precambrian. Testing of the predictions was based on those Precambrian seafloor carbonate precipitates that have been interpreted by others to have formed originally as aragonite. Of 16 examples of Precambrian seafloor aragonite, 14 fall within the 6 periods of aragonite seas predicted for the Late Archean–Proterozoic by the model, 1 falls on the transition between an aragonite sea and calcite sea, and 1 falls in a period of calcite seas. This strong correlation supports the following predictions of the model: (1) Precambrian seawater was a saline NaCl water with Ca > HCO 3 since at least the Late Archean. (2) The major ion compositions of Precambrian seawater chemistry and their secular variations are in the same ranges as those of Phanerozoic seawater. (3) The Mg/Ca mole ratio in seawater has controlled the types of CaCO 3 polymorphs that have precipitated from Earth9s oceans throughout the Phanerozoic and most, if not all, of the Precambrian.

The origin of the Recent non-marine evaporite deposit of Saline Valley, Inyo County, California

Abstract A study of the mineralogy and hydrochemistry of the Recent evaporite deposit in the Saline Valley playa has been carried out. Halite, thenardite mirabilite, glauberite, gypsum, calcite, dolomite, ulexite, analcime and sepiolite have been identified in efflorescences and/or in sands and muds. Inflow is from springs and mountain streams which disappear on reaching the alluvial fans so that the playa is fed by groundwater flow only. These inflow waters are sodium-calcium sulfate-bicarbonate waters while the playa brines (shallow sub-surface, brine table 0′ to −15′) are sodium sulfate-chloride waters which progressively increase in total ion concentration from the margin in toward the center of the playa. Two initial steps in the development of the playa brines are (1) calcite precipitation within the alluvial fans causing a drastic decrease in Ca 2+ and HCO 3 - proportions in the waters reaching the playa, and (2) precipitation of gypsum at the playa edge, which controls the initial SO 4 2− concentration of the brines. Further evaporation simply leads to waters dominated by chloride and alkalis. The distribution of evaporite minerals in the sands and muds of the playa is zonal. From the periphery to the center of the playa, roughly concentric zones carry gypsum; gypsum + glauberite; glauberite; glauberite + halite. This sequence would be produced in proper order by progressive evaporation under equilibrium conditions of certain solutions in the experimental system CaSO 4 -Na 2 SO 4 -NaCl-H 2 O at temperatures between about 10° and 50°C and 1 atm total pressure. Quantitative agreement in composition of experimental and natural brines co-existing with the same assemblages confirms this equilibrium model for the playa evaporites. The model implies that chemical evolution of the natural brines follows a predictable course, one controlled mainly by the bulk composition of the parent water and by the extent of evaporation. The present zonal configuration represents only a stage in the long-range geochemical evolution of the evaporite deposit; outward migration of the existing zones, and addition of a new zone, with time is postulated.

Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater

The mineralogies of most biotic and abiotic carbonates have alternated in synchroneity between the calcite (hexagonal) and aragonite (orthorhombic) polymorphs of CaCO3 throughout the Phanerozoic Eon. These intervals of calcite and aragonite production, or calcite seas and aragonite seas, are thought to be caused primarily by secular variation in the molar magnesium/calcium ratio of seawater (mMg/Ca . 2 5 aragonite 1 high-Mg calcite; mMg/Ca , 2 5 low-Mg calcite), a ratio that has oscillated between 1.0 and 5.2 throughout the Phanerozoic. In laboratory experiments, we show that three species of scleractinian corals, which produce aragonite in modern seawater and which have flourished as important reef builders primarily during aragonite seas of the past, began producing calcite in artificial seawater with an ambient mMg/Ca ratio below that of modern seawater (5.2). The corals produced progressively higher percentages of calcite and calcified at lower rates with further reduction of the ambient mMg/Ca ratio. In artificial seawater of imputed Late Cretaceous composition (mMg/Ca 5 1.0), which favors the precipitation of the calcite polymorph, scleractinian corals produced skeletons containing .30% low-Mg calcite (skeletal mMg/Ca , 0.04). These results indicate that the skeletal mineral used by scleractinian corals is partially determined by seawater chemistry. Furthermore, slow calcification rates, resulting from the production of largely aragonitic skeletons in chemically unfavorable seawater (mMg/Ca , 2), probably contributed to the scleractinians’ diminished reef-building role in the calcite seas of Late Cretaceous and early Cenozoic time.

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.

Seawater chemistry, coccolithophore population growth, and the origin of Cretaceous chalk

The magnesium/calcium ratio (Mg/Ca) and calcium (Ca) con- centration of seawater have oscillated throughout geologic time; our experiments indicate that these variables have strongly influ- enced biomineralization and chalk production by coccolithophores. The high Mg/Ca ratio of modern seawater favors precipitation of high-Mg calcite and/or aragonite. In contrast, the low Mg/Ca ratio of imputed Cretaceous seawater favored precipitation of low-Mg calcite. We have discovered that some coccolithophore species to- day secrete skeletal elements of high-Mg calcite, rather than low- Mg calcite, as conventionally believed. These species incorporated less Mg when the ambient Mg/Ca ratio was lowered, secreting low- Mg calcite in imputed Cretaceous seawater. Calcification stimu- lates coccolithophore population growth by contributing CO2 to photosynthesis. Three extant coccolithophore species multiplied much faster as the composition of ambient seawater was shifted toward that estimated for Cretaceous seas. Two of these species secreted high-Mg calcite in ambient seawater having Mg/Ca . 1, and incorporation of Mg in a calcite crystal inhibits growth. Cal- cification of the third species, which secreted low-Mg calcite at all ambient Mg/Ca ratios, is hindered by the high Mg/Ca ratio and low absolute concentration of Ca of modern seawater. We conclude that the ionic composition of Cretaceous seawater enabled cocco- lithophores to produce massive chalk deposits, and conversely, that the ionic composition of modern seawater inhibits population growth for most extant coccolithophore species, which occupy nu- trient-poor waters and fail to respond to fertilization by nitrate, phosphate, or iron.