Australia: The Land Where Time Began
The Palaeocene-Eocene Thermal Maximum (PETM) – Shallow Marine Response to Climate Change in Salisbury Embayment, USA
The Palaeocene-Eocene Thermal Maximum (PETM) was an interval of extremely warm climatic conditions that caused disruption of marine and terrestrial ecosystems on a global scale. In this study Self-Trail et al. examined the sediments, flora and fauna from an expanded section at Mattawoman Creek-Billingsley Road (MCBR) in Maryland and explored the impact warming had at a nearshore shallow marine site (30-100 m depth of water) in the Salisbury Embayment. It is indicated by observations that the site shifted abruptly from an open marine to a prodelta setting that had increased terrestrial and freshwater input at the onset of the PETM. It is suggested by microfossil biota that there was stratification of the water column and bottom water that was low in oxygen in the Early Eocene. An unusually large bulk carbon isotope shift resulted from the formation of authigenic carbonate through microbial diagenesis, while the magnitude of the corresponding signal from benthic foraminifera is similar to that at other marine sites. This proved that the landwards increase in magnitude of the carbon isotope excursion, measured in bulk sediments, did not result from a near-instantaneous release of 12C-enriched CO2. Self-Trail concluded that the MCBR site records a nearshore marine response to global climate change that can be used as an analogue for modern coastal response to global warming.
The PETM at 55.80 Ma was an interval when there was extreme global warmth that coincided with a massive release of carbon to the ocean-atmosphere system. The identification in sediments of the onset of the PETM is by a carbon isotope excursion (CIE) of approximately -3 to -4 ‰ (e.g., McInerney & Wing, 2011). Earlier studies of the event largely focused on terrestrial or deep-sea sites (e.g. Kennett & Stott, 1991; Koch et al., 1995; Bowen et al., 2014); there are only relatively few shallow marine sections that have been studied. Shallow water sections offer, however, the unique potential to evaluate how the climate anomaly of the PETM impacted coastal marine and terrestrial environments where ecosystems are sensitive to ocean chemistry and physical coastal processes (e.g. Zamagni et al., 2012; Afzal et al., 2011; Steurbaut et al., 2003).
It is especially important to understand the link between terrestrial and marine processes, and the best opportunity to evaluate the local and regional effects of climate change on deposition of sediment, floral and faunal turnover, and geochemistry is inner to outer neritic sections. They provide context between terrestrial continental and shallow marine deposition, which allows for high resolution correlation between the shelf and open ocean sites (Schmitz & Pujalte, 2003). Changes in benthic faunas are especially revealing, as it documents stratification in the water column, continental runoff, and dysoxia (Morsi et al., 2011; Stassen et al., 2012a). Most middle to outer neritic sites in eastern North America (e.g., Bass River, Wilson Lake and South Dover Bridge, to name a few) are in Maryland to New Jersey (e.g., Gibbs et al., 2006; Sluijs et al., 2007; John et al., 2008; Self-Trail et al., 2012; Stassen et al., 2012b), though none of these sections are from the inner part of the shelf.
The onset of the CIE is used globally to correlate the base of the Eocene (Dupuis et al., 2003), and the shape of the CIE anomaly has provided stratigraphically correlative tie points between deep sea, shallow marine, and terrestrial sites (Bains et al., 2003; Röhl et al., 2007). The terrestrial influx and rapid rates of accumulation (Olsson & Wise, 1987; Gibson et al.,, 1993; Gibson & Bybell, 1994; John et al., 2008) complicate the placement of tie points, and potentially the true magnitude of the CIE (Wright & Schaller, 2013). An expanded record of the onset of the CIE is also provided by these rapid accumulation rates, which reveals rapid geochemical and biological changes that are not recorded in deeper marine sections.
In this study Self-Trail et al. examined a suit of cores from the Maryland Coastal Plain region of the Salisbury Embayment at Mattawoman Creek-Billingsley Road (MCBR; N38o36’45.49”, W77o0.2’52.09”) and compared these records from locations that were more distal in the Atlantic Coastal Plain. The MCBR site, which is comprised of 6 shallow cores that were drilled within a 5 m radius (MCBR-1 through MCBR-6), is one of the most landwards marine PETM sites in the Salisbury Embayment that has been investigated, and high rates of sedimentation during the onset of the CIE have provided an unusual level of detail of the progression into the PETM hyperthermal.
According to Self-Trail et al. the objective of this paper was to provide detailed analyses of flora, fauna and sediments from MCBR in order to document the response of the biota to changes taking place in a setting that was river-dominated during the latest Palaeocene and earliest Eocene. They used high-resolution integrated geochemical and biological analyses of the onset of the CIE at MCBR in order to help elucidate the timing of climate events on the land and in the ocean, highlight the differences in how these systems record the timing and magnitude of the CIE and provide evidence for floral and faunal response to oceanographic conditions that were changing. They also note changes in the mineralogy of sediments and discuss their impact on the magnitude and timing of the CIE.
The appearance of planktonic foraminiferal PETM excursion taxa Acarinia sibaiyaensis and A. Africana, which evolved in the latest Palaeocene, as well as A. soldadoensis-sibaiyaensis transitional forms, which differ distinctly from A. multicamerata of Guasti & Speijer, (2008), also documented a sudden change in surface conditions. Near the base of the Marlboro Clay the population of planktic foraminifera is comprised of 77% surface–dwelling, that have symbionts Acarinia, Morozovella and Igorina, which are indicative of warm, nutrient-poor waters and 22% thermocline-dwelling Parasubbotina, Globanomalina, Subbotina and Planorotalites; taxa that lack symbionts and prefer cool, nutrient-rich waters (Shackleton et al., 1985; Pearson et al., 1993; D’Hondt et al., 1994, Kelly et al., 1998). A change in the volume of nutrient-rich water entering the thermocline, and possibly warming, as documented in New Jersey (Babila et al., 2016) may be indicated by thermocline-dwelling taxa that are nonsymbotic and reach 47% at 11.0 m. The ratio difference between symbiotic surface taxa and nonsymbotic taxa, may indicate a change in the volume of water that is nutrient rich into the thermocline, and possibly warming, as has been documented in New Jersey (Babila et al., 2016).
In the latest Palaeocene the assemblage of foramina is dominated by Lenticullina spp. and Bulimna virginiana. It is possibly indicated by large, robust Lenticullina spp. that are characteristic of the Aquia Formation and lowermost Marlboro Clay samples, where preservation is poor and specimens are few or completely absent, the a combination of chemical and mechanical abrasion; the Aquia sand that accumulated slowly was subjected to bioturbation and resuspension.
The assemblage of shallow benthic marine foraminifera from the earliest Eocene was modified by the addition of Pulsiphonina prima, Spiroplectinella laevis and Valvalabamina depressa. As the assemblage becomes dominated by P. prima, B. virginiana, and Epistominella minuta Spiroplectinella laevis and V. depressa disappear upcore. P. prima occurs associated with the seasonal stresses of systems that are dominated by rivers (Stassen et al., 2015), and B. virginiana and E. minuta are related through comparison with species that are well known to an increased though episodic food supply (D’haenens et al., 2012) such as is commonly found near river deltas.
Throughout the MCBR-2 there are rare larvae of bivalves and gastropods as well as adult pteropod specimens (see (Janssen et al., 2016) for discussion of pteropods) which occur sporadically. Almost all pteropods in the deposit are preserved as pyritised moulds, as is often the case in sediments for the Palaeogene, and they are interpreted as drifting into MCBR, as at the present pteropod can only survive in waters of about 200 m or deeper (Janssen et al., 2016). Sometimes larval bivalves and benthic gastropod shells are preserved as pyritised moulds, though aragonitic shells (identified by microdifraction) are present in intervals that display the best preservation of calcitic micro and nannofossils. The identification of larval specimens to finer taxonomic categories is not possible, because of their small size (about 1 m and less), and lack of co-occurring adult specimens to compare them to.
Development of a prodelta system
At MCBR the sedimentary packages are the primary features reflecting episodic deposition of sediment on the shelf and are different from layers that have been described from the cores from Wilson Lake B and Millville (Wright & Schaller, 2013) that are induced by drilling (Pearson & Thomas, 2015). These sediments are mostly similar to prodelta hyperpycnites that were deposited offshore of rivers during periods of high runoff (Carla & Dellapenna, 2014), though sharp-based fining-upwards packages can be generated in shallow marine settings by Storm and deltaic processes. The terrestrial sediment can be carried along the seafloor by turbidity currents and deposited in graded packages within 1 flood event, when the density of sediment-laden river water exceeds that of seawater. Variable internal lamination in deposits that are derived from rivers can be contributed to by pulsed floods and episodic changes in the concentration of the sediment. Striking similarity can be seen between MCBR and deltaic deposits in the modern Atchafalaya (Allison & Neill, 2003) and Brazos (Carla & Dellapenna, 2014) river deltas in North America as well as in deposits of the Eocene Central Basin of Spitsbergen (Plink & Björklund and Steel, 2004) and Cretaceous Interior Seaway of North America (Bhattacharya and MacEachem 2009). This interpretation is consistent with the depositional analogy between the Marlboro Clay and the mobile mud belt of the modern Amazon (Kopp et al., 2009).
The water depth that is required for the deposition of hyperpycnite ranges from 2 to 10 m, depending on the concentration of the river discharge and concentration of the sediment of the hyperpycnal flow (Lamb & Mohrig, 2009). At MCBR benthic foraminiferal species that exhibit ecological preferences towards the inner and middle neritic zone confirm these depth criteria (Stassen et al., 2015. The rare occurrence of the extant Trochammina inflata (less than 1% of the assemblage) and Neoeponides lotus, species which have a shallow palaeodepth preference of 0-30 m (Kellough, 1965, Stassen et al., 2015), is consistent with hyperpycnal transport of sediments from coastal settings. all species constituting 20% or more of the assemblage (in samples with at least 25 specimens) have preferences of water depth of between 20 and 100 m (Stassen et al., 2015) which is sufficient to support the development of a well-defined, though shallow, thermocline (e.g., Cronin & Dowsett, 1990). At nearby South Dover Bridge lack of hyperpycnites, which has faintly though persistently laminated bedding and no well-developed fining-upwards packages (Aleman & Gonzalez et al., 2012) helps to define the distal limit of hyperpycnal flow.
Magnitude of the CIE in coastal sediments
At the base of the Marlboro Clay the -26‰ shift in bulk carbonate carbon isotope values is substantially larger than the -3.7‰ as measured in benthic foraminifera. It is suggested that the bulk carbonate record of MCBR has been altered by the formation of authigenic carbonates during diagenesis, by the magnitude of the foraminiferal shift, which is comparable to that which was documented in the bulk carbonate and/or foraminiferal fraction from the nearby South Dover Bridge site (Self-Trail et al., 2012), sections in New Jersey (John et al., 2008; Makarova et al., 2017) and the deep sea (e.g., Kennett & Stott, 1991). The more negative values from foraminifera altered by the partial replacement of biogenic calcite with authigenic carbonates that incudes siderite, as compared to values from pristine specimens, illustrates this offset.
It is also suggested by observations at MCBR that the increasingly negative bulk CIE values with decreasing palaeodepth that have been documented from cores elsewhere in the Salisbury Embayment (Wright & Schaller, 2013) are most likely due to increasing authigenic carbonate content with proximity to the land and/or source of sediment and not the result of a near instantaneous release of CO2 that is depleted in 13C, as was theorised by Wright & Schaller (2013). Authigenic carbonates with light δ13C values, such as siderite, form under suboxic diagenesis of modern tropical shelves dominated by rivers, as a result of the buildup of alkalinity during anaerobic remineralisation of organic matter that involves abundant oxyhydroxides and the anaerobic oxidation of methane during sulphate reduction (e.g., Aller et al., 2004). The form of magnetite production by magnetotactic bacteria (Kopp et al., 2009) and the presence of polygonal pyrite in between cell walls in dinoflagellate cysts (Edwards, 2020) are evidenced by microbial reduction in the Marlboro Clay. These iron-rich minerals are initially transported with the fluvial muds and recycled within the mobile mud belt (Aller & Blair, 2006). The CIE measured in bulk carbonate is abrupt; probably because it represents a diagenetic front at the base of the mobile belt. Self-Trail et al. infer the true position of the CIE from the values that are measured in benthic foraminifera.
The shallow marine PETM environment
Self-Trail et al. interpreted the increased abundance of benthic foraminifera S. laevis and V. depressa to denote pulsed organic fluxes and episodic dysoxia that is related to the initiation river-induced stratification. These species inhabited a similar environment at the transition out of long-term stratification (Stassen et al., 2015) in New Jersey PETM sections. Coinciding with the true onset of the CIE, was their occurrence at MCBR, as had been determined from benthic foraminifera, there was an increase in the number of siderite grains near the base of the Marlboro Clay, a decrease in the species richness of calcareous nannofossils, and an abrupt increase in the number of fern spores entering the embayment. According to Self-Trail et al. it is likely that pulses of planktic foraminifers reflect periods of low rainfall and a temporary relaxation of stratification in the water column as the environment transitioned into a high-runoff, high sedimentation prodelta. In culmination, it is most likely that these events denote the initiation of sedimentation that was dominated by rivers in combination with the warming of the PETM.
It is suggested by the almost complete disappearance of planktic foraminifera and calcareous nannoplankton above 10.9 m that surface waters became too fresh to support planktic fauna and flora. It is made unlikely by the continued sporadic presence of pteropods, ostracods, and embryonic molluscs in nonbarren samples from above this depth that dissolution is responsible for the flora and fauna from the photic zone.
At MCBR continued warming and stratification of the water column had the result that low oxygen conditions developed. In the upper (red) clay the dinoflagellate Apectodinium homomorphum, that had processes that were incompletely developed or stunted, that is more commonly found in upper (red) clay, probably represents failure to encyst completely; comparable specimens that have been found in low oxygen settings have been called “ontogenetically young” (Sluijs et al., 2005). Similar specimens have been reported from other sites in the Marlboro Clay in Virginia (Witmer, 1987) and were considered to be environmentally influenced, morphological variants. Also, the benthic foraminifer species B. virginiana increases up-core in absolute abundance, reaching almost 100% in many samples. This almost monospecific assemblage is evidence of continued dysoxia; low diversity and the dominance of small taxa that were thin walled such as B. Virginia are characteristic of low oxygen assemblages (Boltrovskoy et al., 1991; Bernhard & Sen Gupta, 1999).
The rarity of larval specimens of benthic molluscs (bivalves and gastropods) also suggests low oxygen conditions and the lack of adult specimens, which are also known from outcrops of the Marlboro Clay (Gibson & Bybell, 1994; Zachos et al., 2006). The oxygen threshold for molluscs is typically between 0.13 and 0.15 ml/L (5-7 μM) (Diaz & Rosenberg, 1995; Levin, 2003). It is suggested by the presence of only larval forms at MCBR that there was a seasonal dysoxic to anoxic seafloor where these larvae were not able to develop to an adult stage, and it is implied by their rarity that these individuals were not produced in situ; rather they were spawned elsewhere on the shelf. Within the interval of the PETM the lack of adult benthic molluscs is not the result of sampling bias, in general, because benthic mollusc beds are common in the Aquia and Nanjemoy sediments in cores from other sites on the Mid-Atlantic Coastal Plain (Ward, 1985).
Episodic prodelta hyperpycnal deposition that is reflected by the laminated packages from MCBR indicate that the shallow shelf on the Atlantic margin may have experienced a significant increase in terrestrial and input of freshwater during the onset the PETM. This change has the potential to impact geochemical as well as biological systems on the shelf. It was hypothesised by Self-Trail et al. that hyperpycnal flow brought fresh sediment-laden water to the floor of the basin episodically, which drove a reverse salinity stratification of the water column as well as turnover of the flora and fauna. Benthic foraminiferal δ18O values began decreasing, and the normal marine photic zone flora (calcareous nannofossils) and fauna (planktonic foraminifera) that were not able to tolerate the conditions at the fresher surface. Eventually, only the sparse benthic foraminifers (as evinced by planispiral (lenticulinid?) foraminiferal linings) and dinoflagellates could tolerate the brackish water that was rich in sediment.
As a result of climate warming increasing seasonality is that it can lead to higher erosion rates and a hydrological cycle that is more active. In the top 5.8 m of core the absence of all calcareous organic matter coincides with a change to a deep red colour of the sediments, and most likely reflects postdepositional oxidation of iron based minerals such as magnetite. In the Salisbury embayment the position of prodeltas and the mouths of ancient rivers can, therefore, be inferred by mapping the predominantly red sections of the Marlboro Clay.
Though the increased kaolinite amounts, such as those observed within the Marlboro Clay, are often cited as evidence of increased precipitation and weathering in tropical climates (Robert & Kennett, 1994; Gibson et al., 2000; 2012) argues that they represent increased physical erosion of kaolinite clays that are pre-existing present in the Salisbury Embayment.
Schmitz et al. (2001) and Thiry & Dupuis (2000) supported this claim, suggesting that the influx of kaolinite at many PETM sites is most likely due to erosion of clay that had formed previously. The culminated sedimentological (i.e., hyperpycnal flow structures, increased terrestrial spores, seed pods and charcoal) and palaeoecological (changeover in planktic and benthic communities) evidence that has been recorded at MCBR support the conclusion that the Marlboro Clay at MCBR represents highly seasonal, possibly monsoonal, precipitation, and episodic transport of sediment to the marine environment.
A variety of factors, that include a paucity of biostratigraphic events and evidence of pulsed, rapid accumulation of sediment that accompanied by hyperpycnal flow, make the calculation of rates of sediment accumulation at MCBR complicated. At MCBR the PETM is truncated, as is evidenced by the continued presence of high percentages of fern spores, which correlates with the fern spore spike that has been documented at South Dover Bridge (Willard et al., 2014), as well as by overlap of Apectodinium augustum and Phythanoperidium crenulatum at the top of the core, 2 dinoflagellates that briefly co-occurred during the early, though not earliest, Eocene at South dover Bridge (Aleman Gonzalez et al., 2012). According to Self-Trail et al. it is possible that the base of the dinoflagellate P. crenulatum can be used as a local marker species in the Salisbury Embayment, occurring later than the first calcareous nannofossil excursion taxa in SDB and at 6.55 m in the MCBR. The thickness of the Marlboro Clay, from the contact with the underlying Aquia Formation to the base of P. crenulatum, is 5.2 m at South Dover Bridge and 5.6 at MCBR. If it is assumed that when P. crenulatum first occurs it is correlative across the Embayment, its position well up into the Marlboro adds further support to the conclusion that the net accumulation rates at the 2 site are of similar order of magnitude, though somewhat greater at MCBR. Also, it is indicated by the preservation of pteropods that at MCBR the sedimentation rate is higher at MCBR than at all other core locations in the Salisbury Embayment. This is consistent with the observation that the only location where pteropods are preserved with their original aragonitic shell material is MCBR, possibly as a result of buffering in coastal waters (Janssen et al., 2016); in neritic environments, rapid burial is typically required by this preservation state.
Limited biostratigraphic and palaeoecological inferences can be made in the lower part of the core, though in the upper part of the core postdepositional dissolution affected much of the calcium carbonate. Following the onset of the CIE at South Dover Bridge (Robison et al., 2014), and across the Salisbury Embayment (Stassen et al., 2015) the abrupt appearance of Anomalinoides acutus and the disappearance of Anomalinoides affinis has not been observed in these basal Marlboro Clay samples. Also, calcareous nannofossil excursion taxa (Discoaster salisburgensis araneus, D. salisburgensis anartios and Rhomboaster spp.) are absent at MCBR below the barren zone. According to Self-Trail et al. it is not likely that A. acutus which dominates the shelf assemblage across the Embayment, would remain totally absent in these samples, while it is possible that the continued occurrence of A. affinis at MCBR is due to environmental conditions that are restricted to this site. It is also not likely that excursion taxa would be absent as a result of environmental controls, as other species of Discoaster (e.g., D. falcatus and D. multiradiatus) are present in this interval, which suggests that water depths were sufficient to support normal calcareous nannofossil assemblages. The first appearance of calcareous nannofossil excursion taxa, based on Agnini et al. (2014), occurs 50,000 years after the onset of the CIE, and therefore the lower metre of MCBR represents less than 50,000 years and possibly considerably less.
Self-Trail et al. suggest that, based on the microfossil and sedimentological evidence above, the basal 2.3 m of the Marlboro Clay sediments at MCBR correlate to within the 1.3 m interval at South Dover Bridge with no CaCO3 (Self-trail et al., 2014) and they estimate that the basal Marlboro Clay accumulated almost twice as fast at MCBR as at South Dover Bridge. This is consistent with proximal to distal changes in rates of sedimentation across shelves that are influenced by rivers, where shallow shelf rates can be higher than outer shelf rates (Kuehl et al., 1986; Dukat & Kuehl, 1995; Sommerfield & Nittrouer, 1999; Carlin &Dellapenna, 2014).
The application of a 4 thousand year minimum of carbon release of the initial PETM (Zeebe et al., 2016) to the onset of the CIE measured in benthic foraminifera over 2.0 m results, however, in a maximum rate of sediment accumulation at MCBR of 50 cm/kyr. The transition of the initial assemblage of foraminifera from cooler, oxygenated mixed layer conditions, to warm, dysoxic conditions, occurs over a 1 m interval within the onset of the CIE. If it was assumed there was a constant rate of accumulation of 50 cm/kyr without reworking, which is not likely, the transition occurred in as little as 2 kyr.
A complex interaction between the marine and terrestrial realms, one that documents the response of a nearshore system to rapid climate change, is recorded by the sediments and microflora and fauna from MCBR. A local deposition and ecological system that was dominated by rivers was initiated at the onset of the CIE that delivered periodic terrestrial sediments in the form of prodeltaic hyperpycnites and freshwater that resulted in a stratified water column. Surface waters became too fresh to support planktonic communities, and bottom waters that were increasingly low in oxygen resulted in turnover of benthic communities. Formation of authigenic carbonates (which included siderite) - and not a large and near instantaneous release of CO2 into the atmosphere – resulted in highly negative C-isotope values in bulk carbonate. Palaeoecological, geochemical, and sedimentological responses to climate warming at MCBR have provided a useful analogue to understanding and predicting the modern near shore response to a climate that is changing.
Self-Trail, J. M., et al. (2017). "Shallow marine response to global climate change during the Paleocene-Eocene Thermal Maximum, Salisbury Embayment, USA." Paleoceanography 32(7): 710-728.
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