Animal evolution, bioturbation, and the sulfate concentration of the oceans

Abstract

As recognized already by Charles Darwin, animals are geobiological agents. Darwin observed that worms aerate and mix soils on a massive scale, aiding in the decomposition of soil organic matter. A similar statement can be made about marine benthic animals. This mixing, also known as bioturbation, not only aides in the decomposition of sedimentary organic material, but as contended here, it has also significantly influenced the chemistry of seawater. In particular, it is proposed that sediment mixing by bioturbating organisms resulted in a severalfold increase in seawater sulfate concentration. For this reason, the evolution of bioturbation is linked to the significant deposition of sulfate evaporate minerals, which is largely a phenomena of the Phanerozoic, the last 542 million years and the time over which animals rose to prominence.Phanerozoic ͉ evaporite ͉ gypsum ͉ sulfate reduction W ith a current concentration of 28 mM, sulfate is the second most abundant anion in seawater. Sulfate enters the ocean mostly from river runoff, with minor contributions from volcanism (1). It leaves the ocean as either pyrite (FeS 2 ) buried in sediments, formed as a product of microbial sulfate reduction, or as sulfate minerals, mostly gypsum (CaSO 4 ⅐5H 2 O), in evaporite deposits (2). Gypsum precipitates before halite (NaCl) and becomes an important evaporite component when the ion product (IP CaSO4 ) between Ca 2ϩ and SO 4 2-in unevaporated seawater exceeds 23 mM 2 (3); presently IP CaSO4 is 280 mM 2. The concentration of Ca 2ϩ has varied between 10 and 40 mM over the last 550 million years (4), and if this range applies through Earth history, IP CaSO4 would exceed 23 mM 2 with relatively low SO 4 2-levels of 0.5 to 2 mM. If sulfate concentrations fall below this level or if IP CaSO4 becomes Ͻ23 mM 2 , the chances for gypsum supersaturation during evaporation of seawater is reduced. The total amount of gypsum deposition from any given parcel of seawater is also limited by sulfate concentration.Previous studies have documented little evidence for gypsum deposition before the Mesoproterozoic (1.6 to 1.0 billion years ago) suggesting reduced seawater sulfate concentrations before this time (5). This analysis is predicated on the assumption that observations of gypsum abundance faithfully represent the original magnitude of gypsum deposition. However, it is known that gypsum is easily dissolved during weathering (6), and alternative approaches have been developed to evaluate the history of gypsum deposition. One of these derives the history of gypsum deposition from the isotope record of sulfate and sulfide ( Fig. 1) (1), with the fraction of the total sulfur leaving the oceans as pyrite given by the equation:

Sulfur isotopes in coal constrain the evolution of the Phanerozoic sulfur cycle

“…The increase in sulfate concentration enabled more active sulfate removal as gypsum in evaporites, and modeling suggests that the pathways of sulfur removal from the oceans have shifted from pyrite burial early in the Paleozoic Era to sulfate evaporite burial in the late Paleozoic Era and thereafter (8)(9)(10). In this view, the influence of the sulfur cycle on oxygen regulation has also decreased through the Phanerozoic Eon as pyrite burial has decreased in magnitude (11).…”

“…Sulfate evaporites are mainly found during the Phanerozoic Eon (although conspicuous deposits are also found during some periods of the Proterozoic Eon, ref. 6), and it has been suggested that their formation on a grand scale was enabled as sulfate concentrations rose above Precambrian levels (7,8). This increase has been linked to sediment mixing and the resultant pyrite oxidation in sediments by bioturbating animals as they came to prominence through the early stages of the Paleozoic Era (8).…”

Decoupling of body-plan diversification and ecological structuring during the Ediacaran–Cambrian transition: evolutionary and geobiological feedbacks

“…Most of these interactions were characterized by the reworking of fine-grained sediments by sediment bulldozers in diffusion-dominated benthic systems [41], as typified by bioturbation in offshore deposits. This style of biogenic reworking was probably conducive to large-scale changes in both the sediment and the water column, including promotion of water fluxes at the sediment -water interface, average deepening of the redox discontinuity surface, release of nitrogen from the sediment, increase in the sedimentwater flux of iron and manganese, and several-fold increase in seawater sulfate concentration [41,42]. By being the primary determinant of oxygen concentration in the sediment, bioturbation may have also influenced the biomass of organisms, the expansion of aerobic bacteria, the rate of organic matter decomposition and the regeneration of nutrients vital for primary productivity, among other aspects [43][44][45].…”

Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age

“…Much of the sulfide produced in today's ocean will not be available for photoautotrophic oxidation, because the sulfide is produced deep within the marine realm, most commonly within sediments. However, in mid-Proterozoic oceans, although the magnitude of primary production may have been smaller (17), sulfate reduction rates would have been similar or higher, as a greater fraction of primary organic matter was not aerobically respired (49). When combined with increased sulfide availability near or within the photic zone (15,50), these conditions enhanced the likelihood that photosynthetic sulfide oxidizers would moderate oxygen levels due to positive feedbacks (Fig.…”