www.deepseadrilling.org, www.odplegacy.org, www.
iodp.org). These sediments have provided a vital resource
for geologists to understand the distribution of marine
microfossils through geological time and to use the
remains of marine microbiota to answer profound questions about Earth’s past.
Summary of methods
The study of marine microfossils, termed marine micropaleontology, requires a well-equipped laboratory to safely
handle the acids and bases used to dissolve large quantities
of rock. Some means of particle concentration (microsieves, heavy liquids, centrifuges) are also desirable. Most
of all, however, either a light microscope (LM) or a strong
binocular scope is essential. Since the optical resolution of
both these instruments is limited, marine microfossils are
commonly examined by scanning electron microscope
(SEM), transmission electron microscope (TEM), or other
sophisticated microscopy techniques including computed
tomography (CT) scanning and synchrotron analysis that
permit 3D internal and external reconstruction of fossils
(Cunningham et al., 2014; Schmidt et al., 2013).
Distribution patterns
The occurrence of marine microfossils in sediments is
largely dependent on the distribution of living populations
and taphonomic factors. The distribution of marine
microbiota may be controlled by the availability of nutrients, sunlight, water temperature, oxygen, and other factors. Following the death of a microorganism, a complex
series of processes is initiated that may eventually allow
its incorporation into sediment. For instance, after the
death of a planktic diatom cell (Figure 3), the skeleton
(termed “frustule”) settles through the water column
toward the seabed. Since marine waters are commonly
undersaturated with respect to silica (e.g., Tappan,
1980), diatom frustules often dissolve in transit through
the water column, and thus only a fraction of those produced reach the deep ocean. Of those diatom frustules that
avoid dissolution during settling, a large portion dissolves
at the water-sediment interface (see summary in
Witkowski et al., 2012). Thus, only a minor fraction of a
planktic diatom assemblage is incorporated into marine
sediments (e.g., Zielinski and Gersonde, 1997).
Calcareous microfossils (Figure 1) in particular can
undergo dissolution at various depths in the ocean. The
carbonate compensation depth (CCD) is an important
threshold within the ocean where the arrival of carbonate
particles such as calcareous microorganism is balanced
by dissolution; in practical terms, calcium carbonate is
not deposited and/or preserved in large abundances below
the CCD, at ~4.6 km in modern oceans (Pälike et al., 2012).
Thus, modern benthic calcareous microbiota such as foraminifers cannot dwell in the deep ocean below ~4.6 km,
and planktic calcareous microorganisms that inhabit surface waters will not be incorporated into deep-sea sediments deposited below the CCD.
As organic-walled microfossils (Figure 4) are predominantly of clay-silt particle size range, their assemblages
are subject to modification by winnowing during sinking
and transport by bottom currents. While one of the benefits of organic-walled microfossils is that they are not
prone to dissolution, they can be affected by oxidation.
The composition of the skeletal wall of organic-walled
microfossils is now known to differ markedly, even down
to intra-generic level within such biological groups as the
dinoflagellates (Bogus et al., 2012). This can have a significant effect on the resistance of the skeleton to oxidation, with most peridinioids being relatively more
volatile and the gonyaulacoids more refractory (e.g.,
Bogus et al., 2014). Thus, exposure to oxygenated water
masses during sinking, and/or long residence times at the
sediment-water interface where sedimentation rates are
low, will bias the composition of an organic-walled microfossil assemblage recovered from a sediment sample (e.g.,
Zonneveld et al., 1997, 2001).
The interplay between the factors described above controls the distribution of marine microbiota in sediments
and the fate of microfossils that underlies the zonal distribution of pelagic sediments in the oceans (“Deep-sea Sediments” by Lyle, this volume). Generally speaking,
wherever the terrigenous sediment input is low and nutrient supply high enough to sustain planktic microbiota in
ocean surface waters, biogenic pelagic sediments will
form on the seabed. At low latitudes, sediments are dominated by calcareous microfossils, predominantly foraminifera (forming the so-called Globigerina ooze that
blankets much of the seafloor) and calcareous
nannofossils, but potentially also the highly dissolutionsusceptible pteropods (a group of tiny aragonitic mollusks) at shallow sites. Siliceous microfossils generally
prevail in sediments deposited at high latitudes and elsewhere in regions of enhanced upwelling. This does not
mean that diatoms or radiolarians do not occur in
low-latitude pelagic sediments but rather that they are
diluted by the more abundant calcareous taxa favored in
warmer waters. Further, high alkalinity of carbonate-rich
sediments ultimately boosts silica dissolution. On the
other hand, cooler waters of high-latitude seas are more
favorable for silica precipitation, which is why pelagic
sediments in the Southern Ocean are impoverished in calcareous microfossils. In locations where the abundance
and preservation of mineralized microfossils are depleted
for these reasons, organic-walled forms such as acritarchs
and dinoflagellates may well be the main groups of marine
microfossils preserved in the sediment record and thus
available for study in sediments deposited in high-latitude
seas (e.g., Eldrett et al., 2004). These patterns are one of
the main drivers of microfossil paleobiogeography
through Earth history.
Marine microfossils through geological time
Earth’s fossil microbiota spans some ~3.2 billion years
(gyr) (Javaux et al., 2010) or longer – see the controversy
470
MARINE MICROFOSSILS
iodp.org). These sediments have provided a vital resource
for geologists to understand the distribution of marine
microfossils through geological time and to use the
remains of marine microbiota to answer profound questions about Earth’s past.
Summary of methods
The study of marine microfossils, termed marine micropaleontology, requires a well-equipped laboratory to safely
handle the acids and bases used to dissolve large quantities
of rock. Some means of particle concentration (microsieves, heavy liquids, centrifuges) are also desirable. Most
of all, however, either a light microscope (LM) or a strong
binocular scope is essential. Since the optical resolution of
both these instruments is limited, marine microfossils are
commonly examined by scanning electron microscope
(SEM), transmission electron microscope (TEM), or other
sophisticated microscopy techniques including computed
tomography (CT) scanning and synchrotron analysis that
permit 3D internal and external reconstruction of fossils
(Cunningham et al., 2014; Schmidt et al., 2013).
Distribution patterns
The occurrence of marine microfossils in sediments is
largely dependent on the distribution of living populations
and taphonomic factors. The distribution of marine
microbiota may be controlled by the availability of nutrients, sunlight, water temperature, oxygen, and other factors. Following the death of a microorganism, a complex
series of processes is initiated that may eventually allow
its incorporation into sediment. For instance, after the
death of a planktic diatom cell (Figure 3), the skeleton
(termed “frustule”) settles through the water column
toward the seabed. Since marine waters are commonly
undersaturated with respect to silica (e.g., Tappan,
1980), diatom frustules often dissolve in transit through
the water column, and thus only a fraction of those produced reach the deep ocean. Of those diatom frustules that
avoid dissolution during settling, a large portion dissolves
at the water-sediment interface (see summary in
Witkowski et al., 2012). Thus, only a minor fraction of a
planktic diatom assemblage is incorporated into marine
sediments (e.g., Zielinski and Gersonde, 1997).
Calcareous microfossils (Figure 1) in particular can
undergo dissolution at various depths in the ocean. The
carbonate compensation depth (CCD) is an important
threshold within the ocean where the arrival of carbonate
particles such as calcareous microorganism is balanced
by dissolution; in practical terms, calcium carbonate is
not deposited and/or preserved in large abundances below
the CCD, at ~4.6 km in modern oceans (Pälike et al., 2012).
Thus, modern benthic calcareous microbiota such as foraminifers cannot dwell in the deep ocean below ~4.6 km,
and planktic calcareous microorganisms that inhabit surface waters will not be incorporated into deep-sea sediments deposited below the CCD.
As organic-walled microfossils (Figure 4) are predominantly of clay-silt particle size range, their assemblages
are subject to modification by winnowing during sinking
and transport by bottom currents. While one of the benefits of organic-walled microfossils is that they are not
prone to dissolution, they can be affected by oxidation.
The composition of the skeletal wall of organic-walled
microfossils is now known to differ markedly, even down
to intra-generic level within such biological groups as the
dinoflagellates (Bogus et al., 2012). This can have a significant effect on the resistance of the skeleton to oxidation, with most peridinioids being relatively more
volatile and the gonyaulacoids more refractory (e.g.,
Bogus et al., 2014). Thus, exposure to oxygenated water
masses during sinking, and/or long residence times at the
sediment-water interface where sedimentation rates are
low, will bias the composition of an organic-walled microfossil assemblage recovered from a sediment sample (e.g.,
Zonneveld et al., 1997, 2001).
The interplay between the factors described above controls the distribution of marine microbiota in sediments
and the fate of microfossils that underlies the zonal distribution of pelagic sediments in the oceans (“Deep-sea Sediments” by Lyle, this volume). Generally speaking,
wherever the terrigenous sediment input is low and nutrient supply high enough to sustain planktic microbiota in
ocean surface waters, biogenic pelagic sediments will
form on the seabed. At low latitudes, sediments are dominated by calcareous microfossils, predominantly foraminifera (forming the so-called Globigerina ooze that
blankets much of the seafloor) and calcareous
nannofossils, but potentially also the highly dissolutionsusceptible pteropods (a group of tiny aragonitic mollusks) at shallow sites. Siliceous microfossils generally
prevail in sediments deposited at high latitudes and elsewhere in regions of enhanced upwelling. This does not
mean that diatoms or radiolarians do not occur in
low-latitude pelagic sediments but rather that they are
diluted by the more abundant calcareous taxa favored in
warmer waters. Further, high alkalinity of carbonate-rich
sediments ultimately boosts silica dissolution. On the
other hand, cooler waters of high-latitude seas are more
favorable for silica precipitation, which is why pelagic
sediments in the Southern Ocean are impoverished in calcareous microfossils. In locations where the abundance
and preservation of mineralized microfossils are depleted
for these reasons, organic-walled forms such as acritarchs
and dinoflagellates may well be the main groups of marine
microfossils preserved in the sediment record and thus
available for study in sediments deposited in high-latitude
seas (e.g., Eldrett et al., 2004). These patterns are one of
the main drivers of microfossil paleobiogeography
through Earth history.
Marine microfossils through geological time
Earth’s fossil microbiota spans some ~3.2 billion years
(gyr) (Javaux et al., 2010) or longer – see the controversy
470
MARINE MICROFOSSILS