WHERE IS THE CHEMOCLINE? PART 4: GEOCHEMISTRY
Welcome to PART 4, the latest installment in our discussion of the Chemocline in ancient basins. So far, we have discussed framboids HERE and pyrite fossil beds HERE and what I think these deposits tell us about water conditions in ancient basins. In keeping with the theme of a holistic approach to understanding basins, I’d like to shift the focus of this post to geochemistry and what it tells us about water column oxygenation.
When making an assessment of bottom water conditions, redox-sensitive trace elements (RSTEs) provide great insight. There is much and more to say about RSTEs but for our purposes, I can keep it brief. Depending on oxygenation of bottom or pore water, RSTEs are said to be conservative (i.e., they remain suspended in solution) or they precipitate out of the water. Some of these elements are associated with primary productivity, for example phosphorus (P) and barium (Ba). Some elements, such as uranium (U), sorb directly to organic matter across the sediment-water interface, while other elements require varying degrees of oxygen depletion: Under aerobic conditions, manganese (Mn) will precipitate, anaerobic-denitrifying conditions lead to the precipitation of vanadium (V) and chromium (Cr), and anaerobic-sulfate reducing conditions results in the precipitation of molybdenum (Mo):
However, care must be taken when interpreting abundances of RSTEs. Phosphorus is often concentrated in apatitic material associated with unconformities, and times of non-deposition. For example, here is the Moonshine Falls phosphate, a pyrite-bearing phosphatic lag at the base of a black shale-filled channel in the Middle Devonian Ludlowville Shale, Hamilton Group:
Under anoxic conditions Ba is conservative and remains in solution, often precipitating when oxygen is introduced to the system. This particular process is evident in upper regressive systems tracts deposits of the Union Springs Formation immediately below the Cherry Valley Limestone. Field, core and cuttings samples are often rich in Ba and barite nodules where clastic influx, and other RSTEs suggest oxygenation of bottom waters:
Perhaps the RSTE enrichment trends I have studied the most are U and Mo. In 2009, Algeo and Tribovillard published a seminal paper on the subject in Chemical Geology titled “Environmental analysis of paleoceanographic systems based on molybdenum-uranium covariation”. The paper looks at the Enrichment of U and Mo in modern oxygen-depleted environments and casts the data in light of basin hydrography.
There are several reasons that U and Mo are useful for assessing redox conditions in ancient basins: 1) detrital concentrations of both elements are low, U ~2.7 ppm and Mo ~ 3.7 ppm; 2) both elements have long residence times in seawater (U ~450 kyr and Mo ~ 780 kyr) and therefore have nearly uniform global seawater concentrations, and 3) both elements are present in low concentrations in marine plankton and enrichment of these elements is the result of authigenic uptake from seawater, a process enhanced by oxygen-deficient conditions. However, there are noteworthy differences in the mechanisms by which U and Mo accumulate in sediment. Uranium uptake occurs across the sediment-water interface where organic matter acts as a sorbent. This process is heavily influenced by sedimentation rate, which largely controls the amount of time organic matter spends in contact with seawater. For example, in the plot below, note that for a give TOC, some samples show elevated U concentrations. These samples occur at or very near to maximum flooding surfaces, that is, sediments which represent the lowest clastic influx in a sequence stratigraphic framework.
As with U, Mo is also removed from seawater across the sediment-water interface under reducing conditions. However, sulfurized organic matter and/or iron sulfides, such as pyrite are the primary sites of Mo uptake. It is noteworthy that the uptake of Mo requires euxinic conditions (free hydrogen sulfide in the water). As a result, enhanced uptake of U relative to Mo may be indicative of less reducing conditions. Molybdenum uptake is also accelerated by Mn-Fe-oxyhydroxide particle shuttles. Molybdenum is absorbed to these particles in the water column which dissolve upon reaching the sediment-water interface, thus depositing Mo in the sediment.
The majority of Marcellus Shale U and Mo enrichment data occur along trends parallel to modern Mo/U molar ratio. The operation of a metal particle shuttle is indicated by the greater enrichment of Mo relative to U at all enrichment values. This further suggests the presence of free hydrogen sulfide in the water column, an argument supported by framboid diameters discussed in the second post in this series. There is no indication of a drawdown in Mo (i.e. the “basin reservoir effect” where the rate of Mo sequestered in sediment exceeds the rate of resupply). Moreover, the excessive enrichment of Mo, 2-5x the modern Mo/U ratio, argues for occasional renewal of Mo to the basin. These observations rule out the existence of a permanently stratified water mass during accumulation of the Marcellus Shale, another observation supported by framboid diameters. Finally, this process appears to be ubiquitous across the Marcellus Shale from studied sections in the southern tier of New York State and extreme northeastern Pennsylvania, to southwest Pennsylvania and northern West Virginia:
Now that we have established the geochemistry of the bottom waters, lets focus on the upper waters. In a recent article published in Geochimica et Cosmochimica Acta, He et al. (2020) discuss the usefulness of I/Ca as a proxy to understand the redox conditions in the upper part of the water column. Like Mo and U, iodine has a uniform concentration in seawater of ~ 0.05 ppm. The concentrations of the two stable inorganic species, iodate and iodide, are controlled by primary productivity and redox conditions in the water column. Iodate is the common form in well oxygenated environments, while iodide is found in anoxic bottom and pore water. However, iodate is the only iodine species incorporated into carbonates. Therefore, a decrease in I/Ca in bulk carbonates has been used to imply oxygen-depletion in upper waters. The study by He et al. (2020) compares I/Ca ratios from Marcellus Shale samples recovered from a core in Yates County, New York to the trace metal data interpreted by Gary Lash and myself in a Marcellus Shale core from Greene County, Pennsylvania. The trends of U and Mo enrichment in the Marcellus are generally mimicked by I/Ca:
While it is difficult to cast I/Ca in absolute values of oxygen depletion, it is significant that the onset of euxinic conditions in the bottom water are accompanied by I/Ca values indicative of a depletion in oxygen in upper waters. Likewise, as U and Mo enrichment decrease, I/Ca increase suggesting that oxygen supply to bottom water is reflected in the surface water as well.
In summary, while I/Ca data is not conclusive, it is significant in that it demonstrates a decrease in oxygen in upper waters at the same time we see a decrease in oxygen in bottom waters. Such data makes it difficult to imagine a scenario where the chemocline is just a few microns above the sediment-water interface.
Thanks for reading along. After the holiday’s I’d like to wrap this series up with a discussion of why all of these studies matter, at least from an economic standpoint. Until then, I hope that you have a very enjoyable and relaxing holiday.