1 February Daily Log
This morning I awoke to see a lush, green coastline with blue mountains behind. We were about 5 km from land, probably the closest we will be to the coast until we return to Port Moresby on 15 February.
We were near the mouth of the Purari River (the location where it meets the sea), and it was dramatic scenery. The picture doesn´t do it justice. Unlike the Fly River which winds a great distance across a broad floodplain, the Purari River descends quickly from the large mountains of New Guinea to the coast (See Geology of New Guinea). The team chose to run Chirp lines in this region to contrast with those near Fly River. The sea floor offshore of the Purari River deepens more quickly than that of the Fly. You can see this the chart because the isobaths (lines which indicate constant water depth) are close together near the Purari than the Fly, indicating that shelf drops off more quickly.
Near the mouth of a river, muddy, freshwater mixes with salty ocean water, and that morning, J.P. took Louisa, Ramsey, Saro and me outside to look at the water surface and see this in action. He pointed out a clear change in ocean´s water color, indicating a location where different water bodies (like river and ocean water) meet. This color change occurs along a jagged line, called a front, commonly marked with white or yellowish bubbles and debris. We were heading along a track line (the ships path) that crossed into the turbid water discharging from the Purari River. By looking at the ship´s meteorological station, we could see that the fresher, turbid Purari River water also was colder. This is probably because much of this water drains from cooler mountain regions. The warmth of the sun continually heats the ocean´s surface waters. River water contains less salt and, thus, is less dense than salty, open-ocean water, so when it enters the sea, it will float along the surface of the ocean in a water mass called a surface plume (easily seen from airplanes). River water also contains nutrients, organic matter, and sediment so it usually appears a different color. The plume we were in this morning looked green rather than brown suggesting that the water contained lots of single-celled plants in addition to sediment. The increase in marine microscopic plants also be identified in the lab by looking at an instrument called the flourometer.
Andrea Ogston, a faculty member from the University of Washington, gave today´s lecture. She talked with us about how sediments (very small rock particles) are transported in the ocean. We discussed the dominant processes in river systems and oceans that move sediments and how they vary from place to place. In rivers, sand-sized particles hop and bounce; this is known as bed load transport. In the oceans, there is a lot of fine-grained sediment, and it moves in suspension as suspended load. If you were a piece of sediment traveling from a river out into the ocean you would take the following course: from rivers, to estuaries, to the surf zone, to the inner shelf, mid shelf and possibly passed to the outer shelf and shelf break onto the continental slope. We identified the dominant processes moving sediment in each area (estuary, etc.), and we also looked at how to measure the velocity of water and bed sheer stress.