Crust-Ocean-Atmosphere

3.2.1 Crust-ocean exchange

Thermal and chemical exchange between the solid earth and oceans, facilitated by hydrothermal fluids, is an important driver of global geochemical and biogeochemical cycles. We need to understand the processes that influence the vigor and extent of hydrothermal exchange. Pursuing this goal requires not only full penetration of a complete crustal section across the Moho and into the shallow mantle at a fast-spreading ridge, but also must be coupled with shallow targeted drilling and in situ experiments elsewhere. Specifically, how are coupled hydrogeological, geochemical, thermal, mechanical, and biological processes linked to the architecture and physical nature of lithosphere below the ocean? Important questions remain about the spatial and temporal scales of hydrothermal fluid movement and the advection of heat and materials. What processes influence the intensity of hydrothermal exchanges in a range of environments and can we develop predictive models for different tectonic/geologic settings? To what degree are hydrological regimes and microbial communities connected in the subseafloor? Recent research has highlighted strong linkages between microbial activity

and hydrothermal fluid-flow (Perner et al., 2009), but the controls on microbial growth, diversity, distribution, survival, and evolution remain poorly understood. All oceanic environments need to be considered from arcs to major ocean basins, ridge crests to the ancient flanks, sediment, basaltic, and serpentine-hosted systems, and important but hitherto unquantified hydrological systems operating on continental margins, ocean islands, and plateaus.

Many questions about axial zone processes can be addressed by the integration of drilling, monitoring, and sampling programs. We lack crucial information on the permeability structure of zero-age crust, although recent progress from seismic monitoring provides unprecedented details of hydrothermal flow patterns (Tolstoy et al., 2008). Ocean drilling offers unparalleled opportunities for examining the controls of fluid pathways and the spatial and temporal variations in permeability and fluid-flow. The hydrogeology of axial hydrothermal systems greatly affects the biological systems that thrive in the ridge environment. We have little knowledge of how microbiological activity in the subseafloor varies spatially and temporally within axial hydrothermal systems.

How is fluid-flow related to biota dispersal and the connectivity of microbial communities? What are the limiting factors (temperature, availability of chemical energy and/or nutrients) for subseafloor life in these environments?

Tackling these questions requires exploratory drilling and sampling, active experiments where environments are perturbed, and measurements, experiments, and sampling from long-term observatories. Although ocean drilling has established innovative approaches to investigate these systems, late stage scheduling decisions often make long-term planning challenging. For drilling crust in axial zones, the hard rock reentry system (HRRS) offers the ability to install casing with reentry capability on sediment-free and sloped seafloor. The HRRS establishes a hole for casing without coring and is a critical tool for hole initiation for deep drilling and borehole observatories.

Sampling shallow crust in axial zones is perhaps best accomplished by the deployment of seabed drill rigs, of which several types are available with capabilities of coring down to 100-150 m subseafloor in 2000-4000 m water depth.

3.2.2 Plate aging − ridge to trench

In regard to the evolution of ocean lithosphere from ridge to trench (Fig. 3.3), we have so far been unable to determine the relative roles of spreading rates, lithospheric architecture, faulting, basement relief, and sedimentation; how do these processes influence patterns and vigor of fluid-flow? How does the crust – and the microbial communities it harbors – transition from the high-temperature axial region to the lower temperature flanks and beyond? How much are the rates of advective heat transport, fluid-flow, microbiological activity, and seafloor alteration changed as a function of plate aging? How does the evolution of oceanic lithosphere influence the dynamics of subduction processes? Systematic transect drilling in crust of different ages, spreading rates, and hydrological state is required to answer these questions, which are critical for developing a comprehensive and accurate picture of mass and heat exchange, present and past.

Figure 3.3 Diagram showing the predicted changes in crustal properties during aging (from Teagle et al., 2009). (A) Parameters that may influence the intensity and style of hydrothermal circulation through the ridge flanks, such as faults, seamounts, basement topography and impermeable sediments.

Arrows indicate heat (red) and fluid (blue) flow. (B) The difference between the average measured conductive heat flow and that predicted from conductive cooling plate models. The calculated global hydrothermal heat flow decreases to zero, on average, by 65 Ma. At this age the crust is typically assumed to be ‗sealed‘ to hydrothermal circulation. (C) How parameters such as basement topography and sediment thickness affect fluid-flow, chemical exchange, and microbe abundance in the crust during the aging process remain undetermined. The hydrological, physical, chemical, and biological evolution of hydrothermal circulation through the ridge flank could be investigated across a ridge flank. (D) The controls on the intensity and style of hydrothermal circulation could be investigated by the measurement of multiple parameters such as porosity, permeability, and alteration mineralogy.

Ultramafic seafloor (20–25% areal extent along slow-spreading ridges) is highly reactive and its presence has profound consequences for plate-scale rheology, crustal composition, and architecture, and implies geochemical and biogeochemical exchanges of global significance (Cannat et al., 2009). Serpentinites are a major source of water in subduction zones and they play a prominent role in the hydrological cycle (Rüpke et al., 2004). Moreover, study of the serpentinization process itself has taken on notable

importance within the ocean lithosphere community with emphases on biogeochemical processes that are inherent where seawater encounters peridotite. Carbonation reactions (Andreani et al., 2009; Kelemen and Matter, 2008) represent a critical and poorly constrained aspect of this and could become an additional focus area that has until very recently not received much attention.

A top priority for understanding crustal aging is a drilling transect of holes through the upper (~600 m) ocean crust along a ‗flow line‘ within the Pacific basin.

Ideally, the transect will be tied to a complete crustal section through the Moho at the young end and a trench-parallel array of sites to document the inventory and variability of material entering the subduction zone at the old end. Studies across a broad, representative range of sites will examine the relationships between hydrothermal circulation, permeability, geochemical alteration, hydrological systematics, and microbial communities as a function of crustal age, as well as enable the development of cross-hole experiments for longer term experimental constraints on biological and hydrological properties of the ocean crust.