Geology Reference
In-Depth Information
vital in supplying pristine records of astronomically forced signals from the
Cenozoic and Late Mesozoic eras. For earlier times, however, pelagic marine
organisms had not yet evolved in sufficient “rock-forming” numbers. For
the early Mesozoic and earlier times, researchers must rely on shallow
marine, hemipelagic, and continental cyclostratigraphy for astronomically
forced paleoclimate data. While continental facies preserve high fidelity
records of astronomical forcing, e.g., the Newark Basin lacustrine rocks
(Olsen & Kent 1996), such facies are in short supply compared with the
marine record. Shallow-marine cyclostratigraphy, principally from carbonate-
rich peritidal facies, is the main source of astronomical forcing and global
climate change data prior to the Jurassic Period (>200 million years ago).
However, in any lithologic, facies-based cyclostratigraphic study, the work
always involves interpretation, both in the identification of a given facies
and in the interpretation of what that facies indicates about the depositional
environment.
To advance the study of cyclostratigraphy, stratigraphers have searched
for techniques that could provide stable and well-behaved paleoclimatic or
paleoenvironmental proxies at high resolution and could be collected over
reasonably thick sedimentary sequences. The Holy Grail would be a simple,
low cost and fairly quick measurement that would be amenable to time
series analysis and require minimal interpretation. For instance, in the
recognition of astronomically forced cycles in the Late Triassic lake sedi-
ments of the Newark Basin, Olsen & Kent (1996) assigned depth ranks to
quantify the depositional environment interpreted from the facies changes
in the rocks. With the construction of a rock magnetic time series, the
facies/depositional environment interpretation could be short-circuited,
and the rock magnetics would directly quantify the paleoenvironmental/
paleoclimate change.
Rock magnetics and rock magnetic cyclostratigraphy can fulfill many
of these needs. Rock magnetic parameters are used in the subdiscipline
of environmental magnetism to detect the ancient depositional environ-
ment. Rock magnetic parameters can measure variations in the concentra-
tion, particle size, and mineralogy of magnetic minerals in a sedimentary
rock. These measurements are relatively quick and, therefore, inexpensive,
so 1000s of samples can be collected and measured for a rock magnetic
cyclostratigraphic study to document magnetic variations at high resolu-
tion. The measurements are also nondestructive, so the samples can be
retained for other nonmagnetic measurements and examination. The
variations in magnetic mineral concentration and particle size can be tied
to changes in the depositional environment and hence to changes in paleo-
climate or paleoenvironment. Since magnetic minerals in Earth's crust
all contain iron, either as oxides, oxyhydroxides, or sulfides, and iron is
the fourth most common element in the crust, magnetic minerals can
sensitively delineate the cycling of this ubiquitous element through Earth's
atmosphere, biosphere, lithosphere, and hydrosphere. Furthermore, very
small concentrations of magnetic minerals (<0.01%) are easily and
