The late Permian biotic crisis had a major impact on marine and terrestrial
environments. Rising
The late Permian extinction event was the most severe global crisis of the
Phanerozoic in terms of both taxonomic loss and ecological impact
(e.g. McGhee et al., 2012). The current consensus
is that the extinction was likely due to global warming and associated
environmental changes caused by
Microfossils of marine algae are excellent recorders of environmental change in the water column. Of these, the organic-walled cysts of dinoflagellates have proven especially useful in palaeoenvironmental studies (e.g. Ellegaard, 2000; Mertens et al., 2009, 2012; Mudie et al., 2001, 2002; Sluijs and Brinkhuis, 2009; de Vernal et al., 2000). While dinocysts are absent or sparse in Palaeozoic deposits, acritarchs are commonly recorded (e.g. Tappan and Loeblich, 1973), even during the late Permian when acritarch diversity was declining (Lei et al., 2013a). Acritarchs are a group of microfossils of organic composition and unknown affinity (Evitt, 1963). Many acritarchs are, however, considered to be phytoplankton, and some are thought to be precursors of modern dinoflagellate cysts (e.g. Lei et al., 2013a; Servais et al., 2004). Several studies in East Greenland have reported fluctuating abundances of acritarchs during the late Permian and Early Triassic (e.g. Balme, 1979; Piasecki, 1984; Stemmerik et al., 2001), but very few late Permian studies have documented the relative abundance of the different genera of aquatic palynomorphs (for example, Shen et al., 2013). Similar to dinocysts, acritarch morphology has been linked to environmental conditions, and acritarchs with longer processes are generally more abundant in more open marine settings (Lei et al., 2012; Stricanne et al., 2004). Both dinocyst and acritarch studies show that salinity might be an important factor that influences cyst morphology (Servais et al., 2004). It is thought that the longer processes in higher salinities stimulate clustering with other cysts or particles in the water column, and thus enhance sinking to the seafloor (Mertens et al., 2009).
The rock record of Jameson Land, East Greenland, has provided key insights
into marine environmental changes during the late Permian extinction event.
Previous work on this section by Twitchett et
al. (2001) showed that collapse of marine and terrestrial ecosystems was
synchronous and took between 10 and 60
To study
the environmental conditions associated with the deposition of these
sediments we look in detail at a short (9
Map of Jameson Land, East Greenland (adapted from Piasecki, 1984). The location of the studied section is indicated with a red circle. The right panel shows the lithology of the studied section and sample depths. (* is the spore peak after Looy et al., 2001; marine collapse after Twitchett et al., 2001).
Samples were collected at the Fiskegrav location of
Stemmerik et al. (2001), an outcrop in East Greenland
(71
The Fiskegrav section shows a transition from the Schuchert Dal Formation
into the Wordie Creek Formation. The Permian–Triassic boundary, defined by
the first occurrence of the conodont
A total of 36 samples were collected from the top 4.5
Change in major groups of palynofacies. AOM: amorphous organic matter. The grey shaded box highlights the biotic crisis as defined by the high spore : pollen ratios, which are indicative for the disappearance of forest communities. Green shading indicates data from bioturbated rocks; grey shading indicates data from laminated rocks. Grey horizontal bar indicates the extinction interval, based on high spore : pollen ratios.
Samples (of ca. 5
The genus
Aquatic palynomorph from the studied interval.
Concentrations of
all aquatic palynomorphs and relative abundances of specific acritarch and
prasinophyte groups. Arrows indicate intervals with low counts (
Size frequency of body diameter
The most abundant organic particles are phytoclasts and amorphous organic
matter (AOM), which together comprise 45 to 90
Although Permian acritarch diversity is considered low, an overview of
Permian phytoplankton by Lei et al. (2013a) showed a richness of about 20–30 genera during most of the Permian,
with acritarchs belonging to the genera
Concentrations of aquatic palynomorphs were on average ca. 4 times higher in
the laminated rocks compared to the bioturbated rocks (Fig. 4). Acritarch
abundance also declines during the extinction event. The majority (25–100
Changes in acritarch abundance and morphology. The left-hand panel summarizes the main groups of aquatic palynomorphs. On the right, body size, process length and process length relative to body size are shown. Green shading indicates data from bioturbated rocks; grey shading indicates data from laminated rocks. The grey bar indicates the extinction interval, based on high spore : pollen ratios. The five small black arrows indicate samples in which the number of specimens used for measurements was low (4–16 specimens).
The
Consistent with the results of Looy et al. (2001), we find a strong increase
in spores, relative to pollen, in the upper Schuchert Dal Formation (Fig. 2).
Since this peak in spores coincides with the interval of marine
ecosystem collapse defined by Twitchett et al. (2001), we define the period
with the highest spore : pollen ratios as the extinction interval.
Twitchett et al. (2001) found large changes
in marine plankton abundance in their study of the Fiskegrav section, with a
bloom just before the disappearance of trace fossils, followed by almost
complete absence of acritarchs immediately after the collapse of the marine
ecosystem. This phytoplankton bloom is not obvious in our higher-resolution
data, and may simply be an artefact of the relatively low-resolution
sampling of Twitchett et al. (2001), but the
number of acritarchs is indeed greatly reduced during the biotic crisis
(Fig. 4). The peak in spore : pollen and decrease in acritarch abundance
occurs in an interval of ca. 0.8
During the start of the biotic crisis, rocks remain bioturbated but within
the upper 0.5
Mettam et al. (2017) studied
redox conditions in the same section and showed that conditions became
potentially anoxic and ferruginous (
The laminated rocks are associated with a higher number of pollen fragments (Fig. 2), which could be an indication for elevated terrestrial organic matter input (e.g. soil erosion), while the higher amount of AOM can be partly explained by terrestrial organic matter input, and partly by increased marine productivity and better preservation of organic matter due to low-oxygen conditions. Increased weathering and run-off are expected consequences of atmospheric and hydrological changes associated with global warming. Algeo and Twitchett (2010) demonstrated that sediment accumulation rates greatly increased in shallow shelf seas in the latest Permian and earliest Triassic, consistent with enhanced weathering and run-off, and Sephton et al. (2005) found evidence for large-scale soil erosion.
If the laminated rocks are indeed a consequence of enhanced run-off, this is
expected to also affect marine environmental conditions, such as a decrease
in (surface) salinity. At the same time, sea-level fluctuations at and near
the boundary need to be considered since these would also affect marine
environmental conditions and could potentially affect salinity. The
Fiskegrav section has preserved aquatic palynomorphs well, and their
diversity, distribution and morphology can provide valuable information on
marine environmental changes. Palaeozoic studies have shown that acritarch
diversity is generally higher in deeper, more distal settings
(e.g. Lei et al., 2012;
Stricanne et al., 2004). In addition, different studies have shown that
In the Fiskegrav section, the decline in diversity from the upper Schuchert
Dal Formation to the Wordie Creek Formation, together with the change in
assemblage from
Thus, during the extinction event the acritarch assemblage changes to a more
typical nearshore assemblage, despite the ongoing sea-level rise. Instead
of a marine regression, the observed shift in acritarch assemblages could be
explained by an increase in run-off, which affects the nearshore environment
by lowering surface water salinity and delivering nutrients, both of which
are found to affect the distribution of modern phytoplankton
(e.g. Bouimetarhan et al., 2009; Devillers and de Vernal, 2000; Pospelova et al.,
2004; Zonneveld et al., 2013). Dinocyst morphology has also been linked to
environmental conditions
(e.g.
Mertens et al., 2009, 2011), with salinity identified as an important factor
influencing process length
(Ellegaard
et al., 2002; Mertens et al., 2011). Similarly, studies of acritarch process
length and palaeoenvironment show that species and individuals with longer
processes are generally found in more offshore locations, while in inshore
settings acritarchs with shorter processes are more abundant
(Lei
et al., 2013b; Servais et al., 2004; Stricanne et al., 2004). The average
process length of specimens belonging to the
The development of widespread anoxia in the late Permian is usually
explained by an expansion of anoxic deep waters onto the shelf
(e.g. Grasby and Beauchamp, 2009). However, increased
run-off and the development of estuarine circulation provide an alternative
explanation for the development of anoxia in some shallow marine
environments. Furthermore, low salinity is known to
negatively impact biomass and size of modern marine invertebrates
(e.g. Westerbom et al., 2002), and,
along with higher temperatures and hypoxia, might also be a contributing
factor in the size reduction of marine organisms recorded in the aftermath
of the late Permian extinction event, as inferred for the bivalve
In addition to salinity, increasing water temperatures can also affect process length in dinocysts (e.g. Mertens et al., 2009, 2012) but this relationship is not always evident (e.g. Ellegaard et al., 2002). Seawater temperature reconstructions for the late Permian and Early Triassic exist mainly from carbonate-rich sections originating in the Tethys Ocean (e.g. Joachimski et al., 2012; Schobben et al., 2014; Sun et al., 2012). These reconstructions indicate increasing temperatures during the extinction event, and maximum temperatures are reached in the Early Triassic (e.g. Joachimski et al., 2012; Kearsey et al., 2009; Schobben et al., 2014; Sun et al., 2012). Since no water temperature reconstructions exist for the Greenland–Norway basin, at this time, it is not possible to exclude the effects of rising water temperatures on the acritarch morphology. Increasing water temperatures might be partly responsible for decreasing process length in acritarchs; however, a rising sea level in combination with rising temperatures is not able to explain the changes in the acritarch assemblages, which indicate a shift from more open marine to nearshore conditions.
Increased run-off cannot only explain the reconstructed changes in palynofacies and acritarch records, but in addition, a reduction in salinity also has significant implications for palaeotemperature reconstructions calculated from the oxygen isotopes of carbonates. Most Permian–Triassic temperature records are from the Tethys or surrounding areas, where monsoon activity has been proposed to increase (Winguth and Winguth, 2012) and thus salinity is expected to change as well. Permian–Triassic oxygen isotope data for palaeotemperature estimates have been derived from brachiopod shells (Kearsey et al., 2009) or conodont apatite (Joachimski et al., 2012; Schobben et al., 2014), with all studies concluding that temperature rose through the extinction event and that Early Triassic seas were “lethally hot” (Sun et al., 2012). Oxygen isotope values partially reflect seawater salinity, although this is generally ignored in palaeotemperature studies because there is currently no independent proxy of salinity change, and so it is assumed that salinity remained constant. If salinity was reduced in shelf seas due to hydrological changes (e.g. enhanced precipitation and run-off), as it appears for East Greenland at least, then all of these studies may have significantly overestimated Early Triassic temperatures. The actual temperature rise associated with the extinction event would have been lower and below “lethal” levels, which is more consistent with fossil evidence that the oceans were not completely devoid of life.
The highest-resolution palynological sampling yet attempted for the
Fiskegrav location of central Jameson Land has shown significant
millennial-scale changes in marine and terrestrial environments. The biotic
crisis, defined by a peak in the spore : pollen ratio, which indicates
widespread disruption and collapse of the terrestrial flora, postdates the
onset of the carbon-isotope excursion. The spore : pollen peak spans 0.8
The data reported in this paper are available in tables in the Supplement.
WMK designed the study. RJT provided material. EEvS analysed the samples for palynofacies and palynomorph content and performed measurements on acritarchs. EEvS wrote the paper with input from WMK and RJT.
The authors declare that they have no conflict of interest.
This work was funded by the Norwegian Research Council, project 234005, The Permian/Triassic evolution of the Timan-Pechora and Barents Sea Basins. We thank Berit L. Berg and Mufak S. Naoroz for technical support. Samples were collected by Richard J. Twitchett, who thanks Gilles Cuny and the Danish National Research Foundation for logistic and financial support. Edited by: Arne Winguth Reviewed by: two anonymous referees