Geology drives science

Science

PRESS RELEASE: It's not always easy for a biologist to concede to a geologist, but whether you want to harvest crabs, catch salmon, watch killer whales or photograph rockfish, the biology of the Salish Sea really is driven by geology. This month's map of East Sound (the body of water) and the land on Orcas Island surrounding it, including Eastsound (the town), has some nice examples of this.

First let's admit there wouldn't even be an East Sound without the glaciers that moved from the north with a thickness of thousands of feet, gouging out a trough between what we now call Mt. Constitution on the east and Mt. Woolard on the west. These glaciers also carved somewhat north-south troughs in the Crow Valley area to the northwest of Mt. Woolard (1).

Glaciers also left a legacy that now controls circulation in East Sound. A receding and melting glacier left a terminal moraine at the mouth of East Sound (2). Look closely and you can spot the dimples caused by tidal scouring as the moraine focuses the deep water currents to the east side of the Sound's opening and restricts overall bottom current flow.

East Sound's relatively flat seafloor (compared to the rocky outcrops to the north of Orcas Island) is common in glacially-formed embayments with little circulation. Incoming tides bring in fine-grained materials and organic matter that are not flushed out but instead settle and produce a soft sediment fill. The few creeks that drain into East Sound also supply materials that are deposited in the Sound, but these tend to be coarser grained and are not the major source of sediment.

Interestingly, acoustic surveys show that the sediment in East Sound is full of methane and hydrogen sulfide gases from buried decomposing organic matter. Because they're shallow and water doesn't circulate well in them, embayments like East Sound tend to get warmer than surrounding areas.

This warmth can cause phytoplankton blooms, which tend to use up dissolved oxygen in the water and make the Sound's bottom waters anoxic, which in turn can result in more organic material getting trapped on the bottom and decomposing to produce gases in an ongoing physical and chemical oceanographic cycle.

Map by G. Greene and J. Aschoff