Subduction
Subduction Erosion
DIATOMS
Arto Miettinen
Norwegian Polar Institute, Fram Centre, Tromsø, Norway
Definition
Diatoms are single-celled, eukaryotic, photosynthetic
algae of the class Bacillariophyceae.
Introduction
Diatoms are microscopic (usually 2–200 mm) algae characterized by a compound silica cell wall, called a frustule
composed of two interlocking thecae, each of which consists of a valve and a series of girdle bands. Diatoms are
the most species-rich group of algae (more than 200 genera
and up to 100,000 species) and are found in almost every
aquatic environment. The diatom taxonomy is based on
the shape and ornamentation of the valves including a
variety of pores, processes, ribs, spines, marginal ridges,
elevations, and other distinguishable features. Both planktonic and benthic forms exist. Further details of diatom
biology can be found in, e.g., Round et al. (1990) and
Not et al. (2012).
Diatoms play a significant role in global climate via the
global carbon cycle. They constitute the major part of phytoplankton and are major primary producers in the oceans
producing as much as 40 % of the annual organic carbon
in the ocean. They also contribute a major fraction of the
downward flux of particulate organic carbon and, thus, a
major fraction of the export of CO 2 to deep seawater
(Hopkinson et al. 2011).
In the oceans, diatoms occur from the tropics to the
poles, but are most abundant in polar to temperate,
nutrient-rich regions, where silicic acid is not a limiting
factor. Sea surface temperature (SST), sea-ice conditions,
light and nutrient levels, stability of the surface water
layer, and grazing are the most important factors determining the distribution and abundance of diatoms in the ocean
surface waters. Diatoms live in the photic zone in the
uppermost surface waters (commonly 0–50 m, max.
200 m), because they are dependent on light for photosynthesis. Therefore, mostly planktonic species occur in the
open ocean and benthic species occur only in the nearshore environments.
Diatom applications
Di/atoms are used extensively in paleoenvironmental
studies, especially in paleoceanography and paleolimnology. For example, diatoms have proved to be an
excellent paleoclimatic tool for SST reconstructions in
the Nordic Seas and the North Atlantic. Diatoms are good
paleoindicators as they are a species-rich group of algae
and living diatoms have specific tolerances to temperature, salinity and pH levels, and other environmental
parameters. The high diversity of diatoms makes them
particularly useful in high-latitude oceans where calcareous microfossils are often poorly preserved. Therefore,
diatoms are one of the foremost tools available in the biostratigraphic studies and paleoclimatic reconstructions. In
this entry, modern diatom-based reconstruction methods
and topical results are summarized from the North Atlantic
region.
Diatom biostratigraphy
Fossil diatom records reveal that the first diatoms, radial
centrics, appeared in the Jurassic, multipolar centrics in
the Early Cretaceous, pennates in the Late Cretaceous,
and the first raphid pennates at 55 Ma in the Paleogene
(Not et al. 2012). Since diatom species have undergone
rapid evolution through the Cenozoic, it is possible to
establish a diatom biostratigraphy for the North Atlantic;
eight Pleistocene diatom datum events were identified
and tied to the oxygen isotope record and paleomagnetic
stratigraphy in studies on sites ODP 919 and ODP
983 (Koç and Flower 1998; Koç et al. 1999) in the subpolar North Atlantic:
• The first occurrence (FO) of Pseudoeunotia doliolus at
1.89 Ma
• The FO of Proboscia curvirostris at 1.53 Ma
• The FO of Neodenticula seminae at 1.25 Ma
• The last occurrence (LO) of Neodenticula seminae at
0.84 Ma
• The LO of Nitzschia fossilis at 0.68 Ma
• The LO of Nitzschia reinholdii at 0.6 Ma
• The LO of Proboscia curvirostris and Thalassiosira
jouseae at 0.3 Ma
This biostratigraphy can be updated by the ninth event,
“the second occurrence (SO) of Neodenticula seminae in
the 1990s.” N. seminae is a marine planktonic
diatom which belongs to the modern assemblage of the
subarctic North Pacific and its high-latitude marginal
seas. The FO of N. seminae in the North Atlantic is an
indicator of the cooling, which started at 1.26 Ma, leading
to the transition from the dominance of 41 ka cycles in the
climate record to the dominance of 100 ka cycles
and intensified Northern Hemisphere glaciations
(Koç et al. 1999). Conditions in the North Atlantic were
too warm for subarctic N. seminae before 1.26 Ma, after
which conditions cooled to subarctic environment favorable for the species. N. seminae thrived in the North Atlantic until conditions turned too severe with perennial
sea-ice cover leading to the disappearance of the species
at 0.84 Ma (Koç et al. 1999). After an absence of more
than 0.8 Ma, N. seminae reappeared in the Labrador Sea
in the late 1990s (Reid et al. 2007) and was found for the
first time in the northern Nordic Seas with a widespread
modern distribution in the mid-2000s (Miettinen
DIATOMS
185
Subduction Erosion
DIATOMS
Arto Miettinen
Norwegian Polar Institute, Fram Centre, Tromsø, Norway
Definition
Diatoms are single-celled, eukaryotic, photosynthetic
algae of the class Bacillariophyceae.
Introduction
Diatoms are microscopic (usually 2–200 mm) algae characterized by a compound silica cell wall, called a frustule
composed of two interlocking thecae, each of which consists of a valve and a series of girdle bands. Diatoms are
the most species-rich group of algae (more than 200 genera
and up to 100,000 species) and are found in almost every
aquatic environment. The diatom taxonomy is based on
the shape and ornamentation of the valves including a
variety of pores, processes, ribs, spines, marginal ridges,
elevations, and other distinguishable features. Both planktonic and benthic forms exist. Further details of diatom
biology can be found in, e.g., Round et al. (1990) and
Not et al. (2012).
Diatoms play a significant role in global climate via the
global carbon cycle. They constitute the major part of phytoplankton and are major primary producers in the oceans
producing as much as 40 % of the annual organic carbon
in the ocean. They also contribute a major fraction of the
downward flux of particulate organic carbon and, thus, a
major fraction of the export of CO 2 to deep seawater
(Hopkinson et al. 2011).
In the oceans, diatoms occur from the tropics to the
poles, but are most abundant in polar to temperate,
nutrient-rich regions, where silicic acid is not a limiting
factor. Sea surface temperature (SST), sea-ice conditions,
light and nutrient levels, stability of the surface water
layer, and grazing are the most important factors determining the distribution and abundance of diatoms in the ocean
surface waters. Diatoms live in the photic zone in the
uppermost surface waters (commonly 0–50 m, max.
200 m), because they are dependent on light for photosynthesis. Therefore, mostly planktonic species occur in the
open ocean and benthic species occur only in the nearshore environments.
Diatom applications
Di/atoms are used extensively in paleoenvironmental
studies, especially in paleoceanography and paleolimnology. For example, diatoms have proved to be an
excellent paleoclimatic tool for SST reconstructions in
the Nordic Seas and the North Atlantic. Diatoms are good
paleoindicators as they are a species-rich group of algae
and living diatoms have specific tolerances to temperature, salinity and pH levels, and other environmental
parameters. The high diversity of diatoms makes them
particularly useful in high-latitude oceans where calcareous microfossils are often poorly preserved. Therefore,
diatoms are one of the foremost tools available in the biostratigraphic studies and paleoclimatic reconstructions. In
this entry, modern diatom-based reconstruction methods
and topical results are summarized from the North Atlantic
region.
Diatom biostratigraphy
Fossil diatom records reveal that the first diatoms, radial
centrics, appeared in the Jurassic, multipolar centrics in
the Early Cretaceous, pennates in the Late Cretaceous,
and the first raphid pennates at 55 Ma in the Paleogene
(Not et al. 2012). Since diatom species have undergone
rapid evolution through the Cenozoic, it is possible to
establish a diatom biostratigraphy for the North Atlantic;
eight Pleistocene diatom datum events were identified
and tied to the oxygen isotope record and paleomagnetic
stratigraphy in studies on sites ODP 919 and ODP
983 (Koç and Flower 1998; Koç et al. 1999) in the subpolar North Atlantic:
• The first occurrence (FO) of Pseudoeunotia doliolus at
1.89 Ma
• The FO of Proboscia curvirostris at 1.53 Ma
• The FO of Neodenticula seminae at 1.25 Ma
• The last occurrence (LO) of Neodenticula seminae at
0.84 Ma
• The LO of Nitzschia fossilis at 0.68 Ma
• The LO of Nitzschia reinholdii at 0.6 Ma
• The LO of Proboscia curvirostris and Thalassiosira
jouseae at 0.3 Ma
This biostratigraphy can be updated by the ninth event,
“the second occurrence (SO) of Neodenticula seminae in
the 1990s.” N. seminae is a marine planktonic
diatom which belongs to the modern assemblage of the
subarctic North Pacific and its high-latitude marginal
seas. The FO of N. seminae in the North Atlantic is an
indicator of the cooling, which started at 1.26 Ma, leading
to the transition from the dominance of 41 ka cycles in the
climate record to the dominance of 100 ka cycles
and intensified Northern Hemisphere glaciations
(Koç et al. 1999). Conditions in the North Atlantic were
too warm for subarctic N. seminae before 1.26 Ma, after
which conditions cooled to subarctic environment favorable for the species. N. seminae thrived in the North Atlantic until conditions turned too severe with perennial
sea-ice cover leading to the disappearance of the species
at 0.84 Ma (Koç et al. 1999). After an absence of more
than 0.8 Ma, N. seminae reappeared in the Labrador Sea
in the late 1990s (Reid et al. 2007) and was found for the
first time in the northern Nordic Seas with a widespread
modern distribution in the mid-2000s (Miettinen
DIATOMS
185