Seamounts 2007 Expedition Background
Research Program #1 - Geological sampling of Patton Escarpment and the seamounts
The California Borderland is a region of complex geology located offshore southern California. It is still evolving geologically and the Transverse Range has high uplift rates and the onshore and offshore areas are riddled with transcurrent faults, many still active and the loci of occasional large and numerous small earthquakes. Our work over the past several years, focused on the outer borderland, where relatively little work has been done, suggests that volcanism played a major role in the evolution of the region. Many people have worked in the inner borderland over many years, but geologic reconstructions have been confused by the abundance of erratic rocks that were recovered in dredges.
We began work in this region several years ago by studying the ages, structure, and compositions of the lavas that make up seamounts in the region just outboard of the borderland. At the same time we undertook a study of the volcanic components in sediments from several DSDP cores (sites 467, 468, and 469) collected near the Patton Escarpment. That preliminary study was recently published (Marsaglia et al., 2006). In it we show that the volcanic rocks and the sediments in the drill core record the southward passage of the Rivera triple junction from 18 to 16 million years ago, and the transfer of the Borderland from the North American to the Pacific Plate at that time. We then completed one exploratory dive (T667) in 2004 that recovered rocks from a section of calc-alkaline lavas interbedded with silicified volcaniclastic sediments. These calc-alkaline lavas turned out to be 17 million years old as well, and to also be part of the record of the passage of the triple junction. In order to place these samples into a broader context we searched the DSDP core locker, and the dredge collections at Scripps and the USGS, where we located a number of samples pertinent to our study on the origin of the region. We have completed chemistry and Ar-Ar age dating on a number of these samples, from several locations, and found that some are about 30-31 Ma old and have compositions most similar to seamounts. These may be seamounts formed on the Pacific Plate that were accreted into the sedimentary prism offshore southern California, only to be subsequently sliced into slivers by transcurrent faulting. At one of these dredge sites, sedimentary rocks include clasts of coarse-grained gabbro and ultramafic rocks that may be the plutonic core of one of these seamounts, now exposed due to faulting.
We plan a 3-dive program to explore more of the Patton Escarpment to determine the extent of these different volcanic units. These sites cover most of the length of the Patton Escarpment and should help us define the boundaries of different terrains that make up the Outer Borderland. The sites are selected based on the availability of modern bathymetric data and the steepness of the escarpment at these sites, since volcanic rock can support steeper slopes than softer sedimentary rocks. In addition, the sites were selected to sample along the entire length of the escarpment. We will also explore a relatively fluid lava flow we discovered last year near the summit of Davidson using the MBARI Mapping AUV. The new high-resolution data shows what appears to be a young fluid flow that partly filled a depression between several volcanic cones. This flow is thin (and therefore fluid) and may represent an eruption much younger than the 9.8 to 14.8 million year old ones we have sampled previously. At San Juan Seamount, where we shall also return late in the cruise, one of the flows we dated erupted 2.7 million years ago, about 16 million years after the oldest dated flow. The duration of volcanic activity on these seamounts is unlike anything seen on other seamounts in the Pacific and is probably tied to tectonic changes along California margin. The seamounts are probably a different part of the same tectonic story as the volcanic history of the Patton Escarpment.
Research Program #2 - Bamboo coral collection
Deep-sea corals are a critical component of benthic ecosystems on seamounts and hardgrounds along the continental margins of the Western U.S. Studies of the biology and ecological importance of these corals are still in their infancy, but they may provide critical biological archives of past deep water environmental change on decadal to centennial time scales. Along the California margin, corals are common at water depths (500-2500 m) where they are bathed by low oxygen, low pH North Pacific Intermediate Water and southern-sourced intermediate water (termed ‘southern component water’, derived from Antarctic Intermediate water and/or South Pacific Mode Water). This community of intermediate depth corals offers a unique opportunity to reconstruct ocean circulation and environmental conditions on short time scales across this critical region of the water column.
Deep ocean circulation (>2000 m depth) is linked to changes in climate via shifts in the density of high latitude source waters, but there is a poorer understanding of the global impact, mechanisms and rates of change of intermediate depth currents (300-2500 m). Intermediate water properties are critical to the earth's climatic system for a number of reasons, including: 1) capacity for transferring heat and salt over vast areas of the ocean; 2) proximity of intermediate waters to atmospheric influence, with a resulting sensitivity to climate change; 3) linkages to the strength of the oxygen minimum zone and related effects on oceanic nutrient distribution, deep-sea ecology and the global carbon cycle; and 4) potential instability of the methane hydrate reservoir controlled by temperature changes at these critical depths.
Geochemical records from the California margin indicate that intermediate water source, temperature, and oxygenation have varied considerably during the past 60,000 years. It has been hypothesized that cold, well oxygenated intermediate waters from the North Pacific (NPIW) were dominant during cool episodes (glacials and rapid, cold stadials), and warmer, oxygen-poor waters from the tropics (‘southern component’ water) dominated during warm episodes, i.e. interstadials and the Holocene. This alternation of intermediate water source appears to occur rapidly and synchronously with or preceding surface water temperature change during major climatic transitions, an indication of the importance of intermediate waters in responding to rapid climate change. There is also growing evidence that methane hydrate destabilization, driven by changes in sea level and/or intermediate depth temperature, may have played a role in driving abrupt climatic changes during the late Quaternary. Therefore, it is of great interest to understand if temperature and geochemical changes occur at intermediate depths on short timescales, for example due to ENSO, Pacific Decadal Oscillation, or anthropogenic forcing, as suggested by limited temperature datasets and models. To address these questions, we need a well-calibrated biological archive of past intermediate water environmental change. Bamboo corals are a group of Octocorals within the order Gorgonacea. These corals have been observed from 300-3000m. Members of this group grow via the production and subsequent thickening of calcite internodes that are separated by organic (proteinaceous) nodes and produce a variable branching morphology. During much of the corals growth, nodes and internodes appear to be precipitated simultaneously, producing temporally-linked organic/inorganic skeletal components. As in many other corals, bamboo corals produce visible banding in their calcium carbonate skeleton. The frequency of the fine-scale banding is likely annual or sub-annual. Most important, carbon in the calcite skeleton is derived primarily from ambient dissolved inorganic carbon whereas organic node carbon appears to come from surface-derived organic matter that serves as the food source for the coral.
Despite the geographic extent and the potential importance of bamboo corals to the benthic deep-sea ecosystem, their biology, geochemistry, biogeography and potential as archives of environmental change remain largely unknown. Previous research on bamboo corals, including our NURP supported pilot study, demonstrates their use as recorders of environmental change from the ocean’s interior. On this cruise we plan to collect bamboo coral specimens from near the California margin, and to conduct geochemical analyses to further develop these archives of temperature, geochemistry, and ventilation history of northern and southern sourced Pacific intermediate waters. We plan to collect living and dead bamboo corals and water samples from a depth transect along the California margin with four objectives:
- Measure radiocarbon (?14C) composition of coral organic nodes and calcite internodes to quantify coral growth rates and changes in the age, flow rate and/or source of intermediate waters with linked comparisons to surface reservoir ages.
- Calibrate skeletal chemical variations (Mg/Ca, Sr/Ca, d18O) with ambient water chemistry and physical parameters to resolve changes in environmental conditions (e.g. temperature, salinity, growth rates) on decadal to centennial timescales.
- Quantify the geochemical environment in which bamboo corals live, including O2, pH, salinity, temperature, and water depth, with implications for understanding the ecology and biogeography of bamboo corals.
- Determine if/where ‘fossil’ corals are preserved along the California margin, based upon coral colonies and calcite saturation, for insight into past processes in the ocean’s interior.