Cold winds, ancient life, and fossil preservation

Sea scorpion fossil from the Soom Shale,
South Africa, ~440 m.y. old
Credit: Univesity of Leicester, as reported
in ScienceDaily, 11/29/2010

Schematic diagram of possible origin of the Soom Shale
from Gabbott et al., 2010, ref. in text

The Soom Shale in South Africa is rich in well-preserved fossils, and geologists have wondered both why they were so abundant, and why they are so well-preserved. In some fossils, you see not only the bone structure, but muscles, gills and swimming paddles.  The answer appears to lie in the story of fierce winds pouring off an ancient glacier, presented in a paper in Geology this month (Gabbott, S.E. et al., Eolian input into the Late Ordovician postglacial Soom Shale, South Africa, Geology, 38(12), 1103-1106.)

During the Hirnantian glacial event, a 1-2 million year event at the end of the Ordovician period which extended from 490-433 million years ago, ice built up over the primitive continent of Gondwana.  his was a period of diverse marine invertebrates and early vertebrates. The ice sheet collapsed rapidly at the end of this time, producing a variety of glacial deposits,topped by the Soom Shale, a 10-15 m-thick black shale. The demise of the marine communities at the end of the Ordovician is widely viewed as the second most devastating extinction to marine life in the history of the earth, and the third largest of the five major extinctions. T Sarah Gabbott and colleagues analyzed thin sections of the shale and found that it contains coarse silt and fine sand in discrete layers (laminae) that are associated with organic material derived from plankton.  They concluded that the geologic setting resembled that found today on the McMurdo Ice Shelf in the Antarctic.  Dust, carried by fierce glacial winds, blew off the ice shelf and into the sea where it fell through the water column.  It carried nutrients that stimulated the production of phytoplankton, which sank to the bottom. In a press release, Gabbott speculated that not only did the nutrients fuel life, they preserved it well when the rotting carcusses sank into the cold, stagnant depths.  "A cold wind, here, was key to both life and death."

A few terms: plankton are the organisms that drift in the oceans, seas or bodies of fresh water.  There are three primary groups, defined according to their function: phytoplankton that use light for photosynthesis (a "producer"); zooplankton that feed on the phytoplankton (a "consumer"); and bacterioplankton (a "recycler"), which basically recycle the other two types of plankton.

Related posts on this blog: katabatic winds and icebergs,
katabatic winds and Mars.