I'm working on two research projects this Fall. I already wrote about one of them, now I'll say a bit about the other.
Following-up on my summer work with the USGS, I'm looking for trends in physical properties of the sediment at the bottom of San Francisco Bay. Trends in time, space, and depth below the "sediment-water interface." The first two are straightforward; it's the latter I want to note right now.
The conceptual model is that there is a line you can draw between sediment at the bottom of the Bay and the water that comprises the Bay. When we take a sediment core (see photo), we start at this line and then take sections from various depths in the sediment column.
A complication in our sampling strategy is that this concept of a clean sediment-water interface is false. The sediments in the bay are all very fine, mostly mud and silt. These are easily picked up by the water, and in certain conditions, they can take a long time to fall back to the bottom. This means that, close to the sediment bed, there is a high concentration of sediment in the water, and the top layer of sediment is very "fluffy". The boundary between the water and sediment is more a gradient, from water, to sediment-rich water, to water-rich sediment, to sediment.
When we scoop up a sample, we often drain water from the core. Ideally, perhaps, we could drain water from the bottom so that any sediment in the water would fall on to the top of the sediment—this collapses some of the gradient zone but keeps all of the sediment present. On a boat and in a hurry, however, it is much easier to drain from the top, which ends up removing some of the fluffiest sediment in this gradient zone. This isn't a grave issue—we are aware of the limitations of our technique—but we are losing some of our most interesting information.
This is a microcosm of an issue that has much bigger ramifications. In low-lying areas, a similar question arises: where do you choose to draw the line between water and land? This becomes a question of political territory, a question of engineering, a question of power. Here's an example from Southern Louisiana, below. The difference between solid ground and water is not so much a firm line, but a threshold that we choose to draw somewhere.
Not one thing but a gradient,
Lukas
Following-up on my summer work with the USGS, I'm looking for trends in physical properties of the sediment at the bottom of San Francisco Bay. Trends in time, space, and depth below the "sediment-water interface." The first two are straightforward; it's the latter I want to note right now.
The conceptual model is that there is a line you can draw between sediment at the bottom of the Bay and the water that comprises the Bay. When we take a sediment core (see photo), we start at this line and then take sections from various depths in the sediment column.
A complication in our sampling strategy is that this concept of a clean sediment-water interface is false. The sediments in the bay are all very fine, mostly mud and silt. These are easily picked up by the water, and in certain conditions, they can take a long time to fall back to the bottom. This means that, close to the sediment bed, there is a high concentration of sediment in the water, and the top layer of sediment is very "fluffy". The boundary between the water and sediment is more a gradient, from water, to sediment-rich water, to water-rich sediment, to sediment.
When we scoop up a sample, we often drain water from the core. Ideally, perhaps, we could drain water from the bottom so that any sediment in the water would fall on to the top of the sediment—this collapses some of the gradient zone but keeps all of the sediment present. On a boat and in a hurry, however, it is much easier to drain from the top, which ends up removing some of the fluffiest sediment in this gradient zone. This isn't a grave issue—we are aware of the limitations of our technique—but we are losing some of our most interesting information.
This is a microcosm of an issue that has much bigger ramifications. In low-lying areas, a similar question arises: where do you choose to draw the line between water and land? This becomes a question of political territory, a question of engineering, a question of power. Here's an example from Southern Louisiana, below. The difference between solid ground and water is not so much a firm line, but a threshold that we choose to draw somewhere.
Not one thing but a gradient,
Lukas