After 8 hours of transit time we have moved from our deepest station at the Peru-Chili trench to our shallowest station on the continental shelf (Station 2). We are 7 miles from the coast of Peru, city lights glow along the night horizon – a small tease of land only one week into a two month cruise. Our depth is around 150 meters so sampling is quick, we will do two similar shallow stations on the shelf within the next two days. Continental margins are interesting locations to study trace metal chemistry because they receive inputs from land and rivers and can exchange metals at the sediment-water interface.
Our stations thus far have been located in a coastal upwelling zone. Winds patterns move air parallel to the coast of Peru pushing surface waters in the same general direction. Friction from surface water pulls on deeper water, but the Coriolis effect causes the direction of flow to deflect to the left in the southern hemisphere. The result is a net westward movement of warm surface waters away from the coast, which is replenished by the upwelling of nutrient rich, cool bottom waters (known as the Ekman spiral, pictured below for the northern hemisphere where the Coriolis effect deflects to the right).
Nutrient rich upwelling zones are biologically productive environments, important to fisheries and filled with wildlife. We’ve spotted three different humpback whales spouting and breaching in the distance (below, if you look close you can see a flipper). Our boat is surrounded by seals at the moment…there’s a group of 15 swimming around in the light from the ship, they are probably accustomed to snagging a free meal from fishing boats in the area.
Above, members of the science team are on deck waiting to deploy sampling equipment, notice all the jackets and hats despite being close to the equator. Water upwelled near the coast comes from the deep Pacific and is cooler than surface waters at the same latitude. Moving westward along the cruise track and out of the upwelling zone, we will eventually encounter more tropical weather.