climatic history. Its effect is that the record of depositional events is smeared in time,
losing resolution rather dramatically. Oceanic sedimentation rates can be extremely
small, as low as 0.1 mm (1000 yr)−1 under the subtropical Pacific gyres. For sites with
such low rates, even with a small L of 4.0 cm, the time at which a particular fossil
arrived in the sediment can only be known within about 400,000 years, longer than
the periods of the glacial–interglacial cycle. Brief events, for example glacial
terminations, are not represented by a plane of abrupt change in the sediment, but by
widely dispersed and upward-skewed vertical distributions, as modeled by Guinasso
and Schink (1975). Moreover, final storage in the stratigraphic record for events of
paleontological or climatic interest only occurs after mixing to the deepest level (L)
represented by tracers of the longest half-life, and is influenced by the very smallest,
most long-term values of DB. Of course, stratigraphy is not hopeless. Something can
be made of changes downward through strata, provided that sufficient care is
exercised. There are sites with much more rapid deposition and sites under nearly
anoxic bottom water, such as the Santa Barbara Basin off California, USA, that have
no bioturbation and sediments showing annual layers termed varves. It is appropriate
that sedimentologists have given very close study to these few sites.
(^) Bioturbation has also been credited with a role in the evolution of global
biogeochemical processes. The notion is that in the late Precambrian, as autotrophs
began to increase oxygen levels in the atmosphere and oceans, some plants (mosses
and liverworts by 700 MYA, fungi and lichens a bit later) colonized land, generating
enhanced erosion that transferred clay particles to the sea. Charged clay surfaces in
water bound organic matter to the particles, increasing the burial of organic carbon,
reducing net relative respiration and, thus, increasing global oxygen levels (Kennedy
et al. 2006). Increased oxygen and buried food supported an adaptive radiation of
marine metazoans that initiated bioturbation. Bioturbators broke up sediment-surface
microbial mats, extended organic-matter distributions deeper, and eventually
established the main features of the modern near-balance between organic-matter
production and its metabolic recycling.
Sediment Sculpting
(^) Animal activities producing bioturbation also change the shape of the sediment
surface. This can be subtle or pronounced. Sediment miners move sediment to the
surface, forming mounds, often with volcano-like shapes (Fig. 14.23). Burrowing sea
cucumbers, Molpadia, make the most dramatic mounds, but many different forms,
particularly polychaetes, echiurids, and enteropneusts (acorn worms) make smaller
bumps. Pits and grooves are formed in many ways. Ampharetid polychaetes excavate
small feeding pits at the ends of their parchment-like tubes. These tubes are partly
vertical in the sediment, partly crossing the surface horizontally to the pit. Any