Heceta Bank is a rocky bank located 55 kilometers (km) off the Oregon coast near Florence, centered on approximately 44°N, 125°W, and is roughly 29 km long and upwards of 13 km wide. Heceta Bank is an area of ecological and oceanographic importance. The unique bathymetric features and seasonal circulation within the bank provides habitat for a diversity of economically-important fish species.
Heceta Bank also plays an important role in current understanding of prehistoric human migration. During the last glacial low stand, approximately 15,000–20,000 years ago, prehistoric humans migrated from northeastern Asia into North America. Scientists hypothesize that Heceta Bank, due to its topography and morphology, is a possible area of this historical migration.
Geomorphology Formation Heceta Bank is one of four commonly-identified offshore rocky banks – including Nehalem, Stonewall, and Coquille – that break up the outer limits of the continental shelf off the central Oregon coast. The banks were formed by the subduction of oceanic plates, which created uplift of the continental crust and exposed much of the rocks found on the banks today. Some of these rocks, particularly those discovered on Heceta Bank, date back to the Pleistocene and pre-late Miocene. The presence of these old rocks, as well as a series of folds and faults underneath the banks, provide evidence for this subduction and uplift phenomenon.
Bathymetry and slope The continental shelf off the Oregon coast ranges from approximately 17 to 74 km in width, from the shoreline to the edge of the continental slope, and reaches a maximum depth of 145 to 183 meters (m) on its outer limits. The shelf has a relatively steep, downward facing seafloor as it transitions into the continental slope. However, due to uplift of the continental crust, Heceta Bank extends the outer limits of the continental shelf and creates an area with relatively shallower bathymetry and less-steep seafloor slope. On its landward side, the bank starts at a depth of 60 m and progressively deepens until it reaches its seaward face, where the bathymetry drops abruptly to 1000 m in depth. The bank consists of a north and south crest, which are shoals where the bathymetry is distinctly shallower than the surrounding seafloor (Fig. 2). These two crests are separated by an expansive area where the seafloor deepens and reaches a depth of approximately 104 m – also called a saddle.
Sedimentary characteristics To the east, Heceta Bank is bound by a steep and smooth continental shelf. In contrast, the bank's topography is relatively complex and irregular. In shallow depths (<100 m), the bank is composed primarily of rock and boulder outcrops, which are separated by small valley-like features. These valleys are presumed to have been created from the erosion of waves during historical periods when sea level was much lower than it is currently. At greater depths, the seafloor transitions to smaller cobbles and fine-grain sediment, such as mud and silt. This complex seafloor provides important habitat for many benthic-dwelling species, such as rockfish and other demersal species. Furthermore, the irregularity in topography, caused by movement due to crustal uplift and transform faults, also allows for the release of methane gas from the sediment, which occurs from <100 to 600 m depth.
Physical oceanography General Oregon coast ocean dynamics Heceta Bank's ocean currents and dynamics are similar to that of the entire Oregon coast. Along the Oregon coast, the California Current is the major Pacific Ocean Current. It runs southward, bringing colder subarctic water down along the West Coast. The Davidson Current is a coastal counter-current that runs northward along the inside of the California Current. Wind forcing and topography play a large role in the circulation and flow of the coastal waters.
The Oregon coast experiences seasonal wind forcing and subsequently operates under two distinct systems: upwelling and downwelling. In the summer months, typically April to September, wind flow is generally southward. These southward flowing winds cause surface currents to move away from the coastline, in a process known as Ekman transport. Thus, upwelling occurs to replace the coastal surface water, bringing cold, nutrient rich water up from the seafloor. During the winter months when winds primarily flow to the north, downwelling becomes the dominant system. Downwelling acts in the opposite manner of upwelling, transporting coastal surface water to depth. The spring transition from the winter downwelling-dominant system to the summer upwelling-dominant system is quick and includes a sudden drop in sea level of around 10 cm in just a few days. Conversely, the fall transition from upwelling back to downwelling, and the resultant rise in sea level, is significantly more gradual. Although upwelling and downwelling are generally seasonably favored, they can occur during either season since they driven by wind direction.
As explored below, the topographic variations unique to Heceta Bank significantly influence how these wind-driven seasonal changes affect this region. This influence is largely because areas with a wider continental shelf respond less strongly to the wind-driven forces.
Summer circulation Seasonal upwelling transports colder, more dense bottom waters up to the surface along the coastline due to the dominant southward wind stress on the coastal waters. The upwelled water brings nutrients up to the surface euphotic zone. Summer surface temperatures of the water range from approximately 7 °C on the coastline up to 14 °C along the outer edge of Heceta Bank. At the same time, high-density bottom water is advected over the bank by northward bottom currents. Relatively less dense water on the outer southern part of Heceta Bank is also present in the bottom density field. Downwelling, although more typical during the winter, has also been observed during the summer months with events lasting 2–10 days. During the summer of 2001, for example, downwelling favorable conditions were present 29% of the time.
Geostrophic jet The upwelling front is accompanied by a geostrophic upwelling jet. During the summer upwelling season, the southward flowing jet off of central Oregon lies mid-shelf at approximately the 100-m isobath. Its near-surface speed averages 0.35 m/s, with individual events exceeding 0.80 m/s. This is faster than the surrounding water velocity. The bathymetry of Heceta Bank causes the upwelling jet to move further offshore as it follows the bottom contours around the bank until it reaches the southern edge at the 200-m isobath. Here, approximately 0.5 Sverdrup of the cold upwelled water is exported out into the deep ocean as the jet cannot follow the continental shelf's abrupt turn back towards the coastline. The amount of materials lost into the deep ocean depends on the velocity of the jet, which is largely determined by the strength of the wind-driven currents.
The movement of the upwelling jet around the seaward face of Heceta Bank causes the inshore side of the bank to have a significantly weaker flow. This sheltered region allows water parcels to remain on the inshore side for a relatively longer period of time. The longer retention time means marine organisms can remain over the bank longer, increasing the risk of hypoxia and aquatic invasive species establishment. In the southern sheltered region of the bank, a northward flow is often observed because of the cyclonic movement of the flow as it adjusts to the deepening water column.
Winter circulation Seasonal downwelling transports warmer, less dense surface waters down to the seafloor as the northward wind stress causes the surface Ekman transport to shift onshore. The downwelling also transports organic matter down to the seafloor from the productive euphotic zone in the surface layer.
The shelf bathymetry and topography play a large role in the dynamics during these winter months as well. During downwelling conditions, there is a northward downwelling jet centered on the 130-m isobath along continental shelf, but as it moves over the bank, it shoals to the 80-m isobath. In contrast to the southward upwelling jet, the northward downwelling jet has a reduced velocity. Therefore, its path around the bank is not as strong and noticeable, resulting in less offshore deflection and no reversed currents, as seen during upwelling conditions. Inshore of the jet is an area of well-mixed water. Overall, the bank is an area of decreased downwelling strength.
Future research There is much room for future research as current understanding of oceanographic dynamics associated with Heceta Bank is growing, but limited. The majority of observed data comes from only two programs. Coastal Ocean Advances in Shelf Transport (COAST) was a 5-year project funded by the National Science Foundation (NSF) as part of the national Coastal Ocean Processes (CoOP) project. The majority of the data was collected in summer of 2001 and winter of 2003. The project involved participants from Oregon State University (OSU), the University of North Carolina at Chapel Hill (UNC) and the Lamont–Doherty Earth Observatory (LDEO). The Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) program has also collected data through moorings and played a large role in expanding knowledge of the Oregon upwelling system of the inner shelf.
One area of research still relatively unexplored is the impacts of biological and physical processes in the formation of hypoxic water in this area. Most of the research of hypoxic events were conducted after 2002, but much remains unknown. Residence times and pathways for dissolved and particulate matter, especially in the wintertime, is another area of research interest.
Chemistry
Methane seeps Methane seeps are located along the downward slope of Heceta Bank, generally clustered in areas known as pockmarks. The pockmarks are formed by the movement of faults, which cause disturbances and conditions within the bedrock to change. Due to uplift, gas solubility in the sediment decreases and gas is ultimately released from the seafloor. The methane gas originates from a mix of thermogenic and biogenic sources, and methane hydrates embedded in the sediment. Thermogenic sources are formed deep within sediment at high temperatures and pressures by thermocatalytic degradation of organic matter. Biogenic sources account for the degradation of organic matter by methanogenic organisms.
Pockmarks