Feast and famine in saline waters

Evaporites and associated brines play a significant role in the deposition, accumulation and subsurface evolution of both the organic matter and its hydrocarbon products as well as related metal sulphide accumulation, throughout eogenesis, mesogenesis telogenesis and metamorphism, Organic matter in modern hypersaline systems tend to accumulate in bottom sediments beneath density-stratified saline water and sediment columns in evaporitic settings where upper portions of layered hydrologies are subject to oscillations in salinity and brine level.

Organic matter is not produced at a constant rate in such systems, rather, it is produced as pulses by a halotolerant community in response to relatively short times when less stressful conditions occur in the upper part of the layered hydrology (Figure). This happens when an upper less-saline water mass forms on top of nutrient-rich brines or within wet mudflats when waters in and on top of the uppermost few millimetres of any microbial mat freshen. The resulting bloom (a time of “feasting”) by halotolerant algae and cyanobacteria is a time characterized by very high levels of organic productivity (Figure A).

Life in saline systems. A) General ecological principles in stressed saline ecosystems. Sigmoidal growth curve of life moving into a newly created niche (as is created by a freshening event setting up a layered brine). As environmental harshness increases due to abiogenic stress factors (such as increasing salinity, osmotic stress and temperature) the level of biogenic stress (predation or grazing decreases allowing a few well adapted species to flourish leading to a decrease in biodiversity and a short-term increase in biomass. B) Schematic summary (“halomancy”) of the interrelationships between habitat stability, environmental stress or adversity, biodegradation, anoxia and source rock potential in an evaporite basin, showing why schizohaline environmentally stressed mesohaline systems tend favour the accumulation of organic matter in bottom sediments.

In mesohaline waters where light penetrates to the bottom, the organic-producing layer is generally the upper algal and bacterial portion of benthic laminated microbialites (typically characterized by elevated numbers of cyanobacteria). In stratified brine columns, the typical producers are the planktonic algal or cyanobacterial community inhabiting the upper freshened water mass. Halotolerant autotrophic microbial communities, living off the remains of this plankton, float mid-water at the halocline of the density-stratified brine columns. Similar aerobic decomposers can also constitute bottom-layer communities in zones of oxygenated oligotrophic waters.Short pulses of extremely high organic productivity during times of freshened mesohaline surface waters in-turn create a high volume of organic detritus settling through the water column and/or construction of benthic microbial mats.

Then, with the end of the freshening event, the ongoing intensely arid climate that characterizes evaporitic depressions means that salinities, temperatures and osmotic stresses increase rapidly in the previously freshened water mass. This leads to a time of mass die-off, in the once flourishing mesohaline community (“famine;” Figure). First, these increasingly salty waters can no longer support haloxene forms. Then halotolerant life dies back and finally, by the halite precipitation stage, only a few halophilic archaea and bacteria remain in the brine column (typically acting as heterotrophs and fermenters). Repeated pulses of organic matter created during a freshening event in a stratified brine column thus create laminated bottom sediment. Hydrogen-rich algal and eubacterial debris is best preserved within sediments where pore waters are anoxic and remain so until the resulting kerogen is sufficiently buried for it to generate liquid and gaseous hydrocarbons (Demaison & Moore, 1980).

Thus the preservation of organics with long-chain hydrocarbons intact is most likely in settings characterized by long-term anoxia (typically inhabited by less efficient decomposers). This tends to occur beneath stratified eutrophic saline brine columns with a permanent anoxic mesohaline bottom water mass and a transiently freshened upper water mass. Preservation is also possible in benthic mats in shallow oligotrophic systems with permanently hypersaline pore waters, but the propensity for periodic bottom oxygenation means preservation potential of oil-prone mats is less likely in this setting, as is also the case in mats of the strandzone.Increasing rates of mineral precipitation at higher salinities means levels of hydrogen-prone organics are highest in sediments deposited in schizohaline waters that remain in the mesohaline field. Once ambient salinities pass into the penesaline and supersaline, the proportion of organics in the sediment becomes insignificant as it is rapidly diluted by large volumes of gypsum and halite (or trona and glauberite in some continental penesaline/supersaline waters).

Feast and famine cycles are a biotal response to environmental salinity conditions that at first are eminently suitable and then increasingly adverse to life. It reflects the typical biological response to rapid niche creation and expansion as the freshened water layer sets up in the evaporitic depression, and is known as the sigmoidal growth response (Figure A). Freshening of surface waters creates a vacant ecological niche into which microbial life rapidly expands, flourishes and then once more dies back as drying or brine concentration shrinks suitable niche space. Long-term stability in any ecosystem leads to domination by “specialist” species that are well adapted, diverse and efficient in terms of energy flow-nutrient utilisation (Figure B). This biodiverse community takes up and recycles carbon very efficiently through the system and the potential for large volumes of carbon passing into the burial realm is low and hence source potential is also low (a good analogy in the marine realm is the ecosystem that typifies a stable tropical coralgal reef).

Pelicans on the edge of a flooded Lake Eyre, South Australia
Beach composed of shells and 20% fish bones, Salton Sea, California

In contrast, a schizohaline system is one defined by high habitat instability, its lack of habitat permanence encourages opportunistic lifestyles (cycles of “famine or feast”). Opportunist species expand rapidly into any suitable niche space, but then just as quickly die back or encyst as conditions become adverse. Such systems are characterised by high biomass pulses and relatively low species diversity, leading to pulses of organics accumulating in the evaporitic depression, with the remains likely to be buried and “pickled” beneath anoxic bottom brines and pore waters, giving such systems a higher source rock potential (Figure B).

Some hypersaline settings are intermediates in that their waters are constantly hypersaline, stressful to most life, but suitable to a few well-adapted species. These regions are usually populated by a few very specialized species (e.g. “high-salt-in” halophiles) that can survive and flourish in conditions where most life dies. They photosynthesise and metabolize successfully in these hypersaline conditions (for example, by living near haloclines), but do so at rates that are much slower than the growth rates attained by halotolerant “opportunists” at times of “feast.” The long-term cycling of organic materials means bottom sediment layers tend to have lower levels of entrained organics preserved compared to the schizohaline-mesohaline.

There are four main geological association where feast and famine conditions during deposition encourage the preservation of elevated levels of hydrogen-prone organic matter, namely; mesohaline platforms and basinwide settings, especially in the early mesohaline stages of salt precipitation. These evaporites source rock association have no modern day counterparts, whereas the mesohaline lacustrine and deep seafloor stratified brine lake associations for source rocks.

Once the enhanced levels of organics stored in an evaporitic host enter the burial realm, other aspects of the evaporite association can further aid in preservation of evaporitic protokerogens. Shallowly buried sediments and their pores beneath hypersaline depositioonal settings are generally filled by anoxic high-salinity pore fluids. This is especially so in a reflux curtain, which tends to slow or prevent the entry of less dense fresher waters. The latter tend to carry communities of the more efficient oxygenic decomposers. Dense, highly-saline reflux curtains below salt beds can persist for tens of millions of years and last to depths that are well into the catagenic realm. This prevents or slows entry of oxidising waters from above or below, other than in local fault or salt wedge edge-focused regions in the basin hydrology. However if the organics are encased in salt, this can slow or prevent the release of catagenic fluids and the evolving organics can remain until the temperatures of thermochemical sulphate reduction and then metamorphosis are reached. If the kerogens remain encased in salt, the lack of any effective permeability can be a negative in terms of hydrocarbon transfer and storage, but a positive in the creation of chloride-rich metal sulphide carriers as fluids are released when halite ultimately dissolves at uppermost diagenetic or lower end metamorphic realm and beyond.