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Questions built on sand

Some of the sediment cores that Liselle studies. Inset is a map showing the location of the Guaymas Basin, the source of the cores.

Concept: Science is ongoing; answering one scientific question frequently leads to additional questions to be investigated.

By Liselle Persad

As part of my graduate thesis project, I am analyzing data from the International Ocean Discovery Program’s Expedition 385. This expedition explored the Guaymas Basin, which is an underwater structure in the Gulf of California. At the basin floor, new crust is forming as two tectonic plates move away from each other. At the same time, a lot of sediment including sand is being deposited there. But where did that sand come from? To answer that question, we geologists (sedimentologists in particular) study what the sand is made of, i.e., its mineral composition. This can reveal whether the sand came from land or elsewhere in the ocean. Over the past 60 years, many studies have documented the compositions of the Guaymas marine sediment types and their distributions. Through this, three sand sources were deduced: the Sonora and Yaqui Deltas in Western Mexico and the Baja California Peninsula. The peninsula is a long, elongated landform that is separated from mainland Mexico by the Gulf of California. This landform has many desert areas which serve as sources for wind-blown sand. Deltas are a part of a river system and function to move sediment from the land to the ocean. In this case, they serve as the favored sources for the sand observed in the Gulf of California.

That is helpful to know, but left me (and many other scientists!) wondering how the sand actually got there. Were these deposits transported as a result of a hurricane, earthquake, or sudden sediment grain flows. To answer this, I am studying cores (cylinders of sediment pulled from deep in the ocean floor) recovered from eight additional sites in the basin. We’ve already seen that careful analysis of the sand composition can reveal where it came from. And I’m hoping it can also teach us about modes of transport and deposition. Of course, whatever I learn is likely to inspire even more questions. When did these deposition events occur? Where did the sediment come from? What happened to the sediment pre- and post-deposition? Was this sediment hydrothermally altered and what new minerals exist as a result of this? The story lies in the correlation of the sediment itself; macroscopically and microscopically.

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