On Marine Silicates
An emphasis of my early research was on the elucidation of the origin of authigenic and diagenetic marine deposits. The earliest major work focused on the origin of authigenic feldspars and of zeolites in oceanic sediments and implications for chemical paleoceanography. The criteria for uniquely identifying authigenic versus igneous and/or metamorphic feldspars were established. They were reaffirmed repeatedly and are the ones used since.
I then focused on determining the physical-chemical controls on the silica system in the ocean, in particular on the silica phase transformations: opal-A to opal-CT and to quartz in marine sediments, of crucial importance to the formation of the siliceous sediments in the marine environment and the implications for paleoceanographic interpretations of these sediments. The feldspars and zeolites mostly occur in the tropical-subtropical regions, and the silica deposits are the most important sediments in the southern ocean, and are also present in the equatorial regions and continental margin upwelling systems.
Understanding these deposits is key for evaluating ocean circulation and productivity, for example, during glacial/interglacial cycles.
Marine and Authigenic Carbonates
Carbonate sediments are the most important sediments for paleoceanographic studies.
The work on the Sr distribution coefficient was groundbreaking, was crucial in establishing Sr concentrations in calcite as a widely used indicator for carbonate recrystallization, and a fundamental indicator for all paleoclimate studies that depend on carbonate proxies.
The research on carbonate sediments also proved that the transformation of calcareous ooze to chalk and limestone does not require hundreds of pore volumes of fresh-water flow through the system, but is an reaction that occurs in the oceanic environment by in situ sediment-pore fluid reactions, which has fundamental implications for sub-seafloor hydrology.
For the first time it was demonstrated in my laboratory that dolomite (a Ca-Mg carbonate) formation is controlled by its associated pore-fluid geochemistry, solving what had been a nagging problem in carbonate mineral science for over the past ~100 years. This new finding provided a new understanding of the distribution of dolomite versus calcite in the geological record with implications for the history of seawater Ca and particularly of Mg concentrations, thus, on continents weathering intensity and continent/ocean interactions.
The research on marine phosphate deposits completely revised ideas on the stability of P-O bonds in the phosphate ions apatite and led to a recalculation of the ocean residence time of phosphate. This research has significant implications on the oceanic phosphorus cycle, hence on marine biology, as P is a limiting nutrient, as well as on previous interpretations of the paleo-ocean temperature.
Proved that the previous extensively published claim that the PO4-5 ion is ‘immune’ to diagenesis, and therefore marine phosphate oxygen isotopes in apatite could be used as reliable recorders of the paleo-ocean oxygen isotopes and temperatures, is incorrect; that bacteria rapidly re-set the oxygen isotopes in the PO4-5. Instead, the δ18O of the phosphate ion in marine apatites may be used for paleo-diagenesis.
Documented and determined the formation and origin of a modern phosphorite, offshore Baja CA, and recalculated P residence time.
With a Ph.D. student and then a postdoctoral fellow, A Paytan, initiated and showed the importance and reliability of marine barite as a new highly reliable recorder of seawater chemistry, (for example of seawater 87Sr/86Sr) and paleo-productivity, and produced the first continuous high resolution seawater S-isotopes records, over the past 120 million years, using marine barite. This is a prime example of using the combination of mineralogical and geochemical expertise to solving important scientific problems. The oceanic sulfur isotopes record is directly linked to the history of atmospheric and oceanic oxygen levels. Since, A. Paytan greatly expanded the research on marine barite.
Published the first paper on the hydrothermal system in the Guaymas Basin Gulf of California, discovered through ocean drilling, and was involved in the expedition that found the first hydrothermal springs at from 21° N East Pacific Rise and associated sulfide deposits; deciphered the mineralogy of these first ever recovered hydrothermal sulfide chimneys from a ridge-crest. Based on these data the step-wise evolution of such chimneys was established.
At Guaymas Basin in the Gulf of California, the formation of talc on the seafloor was identified, and based on its d18O and dD values I analyzed, an origin at 300°C fluid temperature and a fluid with positive d18O value of ~1.5‰ were established. As the whole ocean cycles through the submarine hydrothermal systems once per few million years (1-5 m.y., depending on the cycle used for the calculations) this process has a fundamental impact on the chemistry of seawater, hence on the atmosphere, as well as on marine biology.
Fluid Flow Monitoring, Technology, and Instrumentation
Fluid cycling in subduction zones and on ridge flanks still are two marine geochemical frontiers, having profound implications for ocean chemistry, biology, understanding earthquake cycles in subduction zones, and continental crust formation. Therefore, it was exciting to be one of the pioneers in this field.
Provided the first evidence for large-scale lateral advection of seawater through the oceanic crust, in the Central Equatorial Pacific, and subsequently offshore Costa Rica.
Was one of the earliest scientists that spearheaded research on the important role fluids have in subduction zones, in particular on geochemical cycling in the forearc, and the significance of some solute fluxes into the ocean, such as Li or Sr isotopes. I was an early advocate of large-scale in situ experiments. For this I designed, and with the help of an engineer constructed, the first continuous long-term high-resolution fluid sampler for a borehole observatory (CORK), that utilized osmotic pumps. It was successfully deployed in a borehole observatory offshore the Barbados subduction zone (ODP Site 948). For this, I adopted the original idea by Hans Jannasch, at MBARI, that osmotic pumps could be used in the ocean instead of battery-powered instruments. The osmotic pump, originally developed by Jannasch, was used by him only forin situ analysis of nutrients in the water column, and each chemical component required a special pump, also it was applicable only to components that can be analyzed in situ colorimetrically, thus was limited to the few nutrient components.
In my new application I demonstrated that, by using very long teflon and/or Cu capillary coils for gases, with length depending on the time of deployment, resolution needed, and thermal gradient, long-term high-resolution time-series records of fluid chemistry and isotope ratios could be obtained, and upon recovery analyzed. Since this first deployment in the borehole offshore Barbados, this approach was adopted enthusiastically by the oceanographic community and it has become the state of the art of fluid sampling in boreholes; and in recent years microbiologists as well adopted this technique for research of the deep biosphere.
The contributions of the subsurface hydrology in the forearc to the formation of gas hydrates, was another line of research, however, the gas hydrate research spanned over both convergent and divergent margins (e.g., the Gulf of Mexico). I documented and explained the role of gas hydrates on the ubiquitous low-Cl fluids observed in convergent margin pore fluids. For this A. Spivack developed the Cl stable isotope methodology for small fluid volumes.
On Chlorine Isotopes and Oceanic Mass Balance
Explained the meaning of the ubiquitous low-Cl fluids observed in convergent margins. For this, together with A. Spivack and a joint Ph.D. student, we developed the Cl stable isotope systematics in subduction zones and demonstrated that chloride is NOT behaving conservatively in this environment. We also clearly indicated with the Cl isotopes in the low-Cl fluids, that the low-Cl is not a simple signal reflecting dehydration of hydrous silicates, but a mixed signal of Cl-uptake by the formation of elevated temperatures hydrous silicates (illite, chlorite, talc, or quantitatively most importantly serpentine) plus dehydration. The Cl isotopes allow to calculate the end member mixture.
The first manuscript on the oceanic budget of Cl isotopes, based on field and experimental data was published by our students as the leading author (W. Wei) in Earth Planetary Science Letters.
On Gas Hydrates
- Evaluated the importance of convergent versus divergent margins in the global methane hydrate budget.
- Characterized chemically and isotopically the first natural CH4-H2S hydrate, previously predicted to occur in the natural environment.
- Documented the flux of methane across the ocean/atmosphere in the northern Gulf of Mexico in a gas hydrate field, recently published in Nature Geoscience. The contribution of deep water (>200m) natural oceanic methane seeps to the atmospheric methane budget is being further pursued.
- Synthesized the natural occurrence of gas hydrates in various continental margins and the origin of the fluids responsible for their formation (see above on fluid flow).