An Introduction to Environmental Chemistry

The Oceans 203

shallow, saline waters where these deposits

are found favour increased concentrations

of carbonate ions (CO 32 - ), increasing the ion

activity product of aCa^2 +.aCO 32 - such that

precipitation of CaCO 3 occurs. The formation

of ooids probably requires fairly agitated,

wind- or wave-stirred waters, allowing

periodic suspension of the grains into the

CaCO 3 -saturated water, whereas aragonitic

needles may precipitate as clouds of

suspended particles, known as whitings.

There has been, and still is, much debate

about the origins of these particles. Firstly, it

is difficult to disprove the effects of microbial

mediation in their formation. Thus we might

regard the grains as non-skeletal, while

accepting a possible microbial influence.

Secondly, various geochemical and

mineralogical studies have produced

equivocal results in attempting to

demonstrate an abiological origin. Having

said this, recent work based on crystal

morphology and strontium substitution lends

support for inorganic precipitation of needle

muds.

Despite the considerable interest these

phenomena provoke, we should remember

that they are of minor significance to the

modern oceanic CaCO 3 budget. The relative

importance of inorganic CaCO 3 in the

geological past is more difficult to assess, but

may have been more significant before the

evolution of shelly organisms about 570

million years ago.

* Aragonite and calcite are known as polymorphs of

CaCO 3. Both minerals have the formula CaCO 3 but

they differ slightly in the structural arrangement of

atoms.

Fig. 6.9The present-day distribution of the principal types of marine sediments. After Davies and Gorsline
(1976), with kind permission from R. Chester.