Earth’s environment in the Palaeocene: A literature review CHARLIE KENZIE

Department of Earth Sciences, University of Durham 2014

1. Introduction

The Palaeocene epoch follows the termination of the Upper Cretaceous and precedes the

Eocene epoch, occupying a timescale from approximately 66-56Ma before present. The

Palaeocene also marks the first, or lower most division, of the Paleogene epoch. The

boundary between the Cretaceous and the Palaeocene (K-Pg) is marked clearly in the fossil

record, and is also palpable by anomalously high iridium levels and ejecta deposits. This

dramatic change in the paleorecord coincides with the mass extinction event at the end of the

Cretaceous. Climatic and environemtnal changes during the Palaeocene are often indicated by

the use of stable isotopes in sediment sections and also data from foraminifera and fossilised

vegetation. Climate change inferred from the above constraints, generally suggest a relatively

cooler early Palaeocene, and warming towards the late Palaeocene. The end of the Palaeocene

is dominated by significant and extreme rises in global temperature and oceanic carbon levels,

and is often termed the Palaeocene-Eocene Thermal Maximum. This short time period has

been the centre of intense scientific research due to its similarity to modern day global

warming, and could be discussed to great length in its own right, and although trends of

isotopic data, environment and climate are explored up to Eocene boundary, a distinct treatise

on the PETM is not within the limits of this review.

The following literature review aims to summarise the literature of the Paleaocene epoch,

focusing on earth’s system and climate using published data and interpretations. As such,

interpretations made by the author are only in an evaluative sense, to ‘critique’ the literature

to an extent. This short review aims to discuss the change in Earth’s processes during the

Paleocne and thus, the literature is discussed by order of these different environmental

systems. A more general discussion of climatic and environmental changes over the whole

epoch, and the potential uncertainties of investigating paleoclimate finalise the review, with

an ending note on the future avenues of research into Palaeocene climate and systems.

2. Ocean Circulation

During the late Cretaceous and early Tertiary the large Pangean land mass continued to break

up, resulting in new ocean gateways and currents. By the start of the Palaeocene large

equator-pole currents were circulating in the Pacific (Frakes et al 1992) and a more limited

circulation was apparent in the South Atlantic. However, a still relatively warm climate over

the Cretaceous-Palaeocene boundary is suggested to have resulted in warm ‘sluggish’

circulation (Frakes et al 1992; Kennett & Stott 1990, 1991; Quillévéré et al 2002). Since

high-latitude oceans were relatively warm, the equator-to-pole temperature gradient was low,

Figure 1. Black line is the five-point moving average of 1209 Palaeocene benthic foraminiferal data from ODP site 1209, a complete high-resolution benthic stable δ18O and δ13C isotope record for the central Pacific (Westerhold et al 2011). Other points shown are the composite multisite deep-sea benthic foraminiferal stable isotope data. NAIP shows the primary component stages of the North Atlantic Igneous Province. Stages of the Deccan Trap igneous province are also shown at the K-Pg boundary. To some extent, the phases of Deccan eruptions support hypothesis by Kellar (2005) that an extinction event at the K-Pg boundary is unrelated to the Chicxulub impact and that Iridium anomalies, climate and environmental change are associated with Deccan traps volcanism. Several small ‘hypothermals’ are inferred by the data (purple lines) but these are yet to be resolved to represent global climate change. During high δ13C periods the climate cooled, whilst during low δ13C periods the climate warmed.

resulting in weak atmospheric and ocean circulation. Low water turnover and oceanic mixing

in this period was compounded by the K-Pg extinction event, which caused “Strangelove”

conditions (Hsu 1985). This is recorded by several studies as a sudden loss of the ocean’s

carbon-isotope gradient (Charisi & Schmitz 1994; Frakes et al 1992; Kennett & Stott 1990;

Kump 1991; Quillévéré et al 2002; Fig.1), and widespread isotopic homogeneity. It has been

suggested that this homogeneity is caused by the nearly complete cessation of primary

production in the surface ocean, brought about by a drop in the net export of organic carbon

from the surface ocean to the deep ocean. Thus, an isotopically homogeneous ocean indicates

a deceleration of ocean mixing (Kump 1991, and references therein).

During the late Palaeocene hydrothermal activity (Vogt, 1979) and ocean mixing has been

suggested to increase. (Miller et al 1987; Kennett & Stott 1990, 1991) have suggested that

oceanic mixing, brought about by major tectonic reorganization towards the end of the

Palaeocene, caused warm saline and oxygen-deficient deep water to mix with cold, nutrient-

depleted deep waters at higher latitudes, thus shifting the locus of ongoing deep water

formation from cold to warm waters, causing deep ocean water temperatures to rise

dramatically. This is apparent from deep-sea benthic foraminifera collected at Cape Basin

(Miller et al 1987). δ18O data suggests bottom water warming of approximately 6°C, and an

inferred change in water supply to that from an Antarctic source, causes or at least

contributes, to the demise of the Palaeocene deep-sea fauna, a commonly measured major

biotic crisis (Miller et al 1987; Kennett & Stott 1990, 1991; Vogt 1979), and interpret these

observations as an important climatic transition (Fig.1). Vogt (1979) also proposes that

increased hydrothermal activity and volcanogenic upwelling of anoxic, nutrient rich water

during this time also contributed to fauna extinction. During the late Palaeocene at high

temperature maxima, the lack of cooled near surface waters emanating from polar regions

allowed penetration of warmed boundary currents to higher latitudes (Boersma & Permoli-

Silva 1983). It is likely that changes in ocean circulation were concurrent, and cooperative

with, global atmospheric and oceanic temperature change.

3. Ocean temperatures

The latitudinal movement of foraminifera can determine intervals of warming and cooling in

the oceans. In cooler times warmth-loving foraminifera are restricted to lower latitudes (about

25°). However, during the late Palaeocene, the same foraminifera migrated into much higher

latitudes of up to about 55°, suggesting that ocean waters warmed at this time (Boersma &

Permoli-Silva 1983). (Shackleton et al 1986) suggest that ocean bottom waters stayed

relatively constant throughout the Palaeocene, between 10 and 12°C, which is in contrast to

observations made by Miller et al (1987) at Cape Basin, who advocate that ocean bottom

waters warmed by 6°C. However, Miller et al (1987) admit that this 6°C warming may not

have reached its maximum during the Palaeocene. Records of ice-rafted deposits are few and

far between during the whole of the Palaeocene. However, a study by Dallan (1977) observed

Figure 2. Integrated stratigraphy and geochemistry across the K-Pg boundary in the El Kef GSSP section (see Schulte et al 2010, and references therein). All data shows a very sudden ‘jump’ at the K-Pg boundary, strongly supporting an impact event like that hypothesised for the Chicxulub crater. Taken from Schulte et al (2010).

Palaeocene clasts of fine-grained shales on Spisbergen (Arctic Norway), and interpreted them

as ice-raft deposits. It has been suggested that, following the K-Pg extinction event, slightly

cooler temperatures during the early Palaeocene may have accommodated seasonal ice.

The literature still debates as to whether there was a gradual disappearance of Cretaceous

biotas (Frakes et al 1992; Kauffman 1984; Officer et al 1987; Keller 2005) or that sudden

catastrophic extinction in oceans and on land occurred due to a meteorite impact event

(Barnosky et al 2011; Hsu 1986; Schulte et al 2010). In the oceans, the surface waters seem to

have been the most affected, since planktonic microbiotas declined markedly whereas benthic

biotas were hardly changed (Frakes et al 1992, and references therein).

4. Cretaceous-Palaeocene extinction event

Since this topic dominates a large proportion of the literature reviewed, and the debate

continues as to the cause of the K-Pg extinction, it is discussed explicitly in this section. The

definition of the K-Pg boundary, as described by several studies (Barnosky et al 2011;

Schulte et al 2010), is that the stratigraphy at the base of the Danian is characterised by a dark

clay bed which shows the coincidence of the mass extinction of marine plankton, ecological

disruption at the sea floor, a drop in carbonate content, and a perturbation of the global carbon

cycle (at the level of hypothesised impact, Fig.2). This perturbation is characterised by

anomalously high Iridium levels, ejecta spherules and Ni-rich spinel. Several studies (Alvarez

et al 1984; Barnosky et al 2011; Hsu 1986; Schulte et al 2010) have attributed the timing,

palaeontology and petrology of this distinctive boundary to a potential meteoroid impact at

the Chicxulub crater, near modern day Mexico. The petrography, composition and age of the

ejecta material present in many of the K-Pg boundary localities match the suite of target rocks

within the Chicxulub crater (Schulte et al 2010).

Conversely, some studies (Dingus 1984, Dingus et al 2000; Kauffman 1984) suggest that

complete stratiagraphic records of mass extinctions of this duration are rare. This

complication is compounded by a period of falling eustatic sea level during mass extinction,

thus enhancing the non-deposition of, restriction of, and erosion of epicontinental and

continental marginal marine sections (Kauffman 1984). With these considerations in mind,

further re-examination of the best stratiagraphic sections by Alvarez & Kauffman (1984)

suggest that terminal-Cretaceous extinction occurred on two timescales. Firstly,

paleontological data showed gradual declines in diversity of many invertebrate groups over a

period of 1-10Myr after the hypothesised boundary impact. Second, that simultaneously to

this gradual decline, four groups (ammonites, cheilostomate beyozoans, brachiopods and

bivalves) were affected by sudden truncations precisely after the impact time. Thus, Alvarez

et al 1984 suggest that some animals declined slowly, unreleated to impact, and that others

declined quickly, synchronously to the iridium anomaly deposits, and were probably caused

by impact.

More recent research by Keller (2005) suggests that K-Pg extinction and other biotic effects

were completely unrelated to the Chicxulub impact event, arguing that Planktic foraminifera

extinction, which suffered the most dramatic decline at the K-Pg boundary, declined slowly

rather than truncating rapidly as would be expected if related to the Chicxulub impact. The

research also draws on previous impact craters of similar size. Keller (2005) argues that no

significant biotic or environmental effects can observed as a result of past impacts and puts

forward an alternative theory for K-Pg mass extinction, which calls for a re-evaluation of the

Chicxulub impact theory. Keller explains the enhanced Iriduium deposits as a result of

volcanism, arguing that recent data suggests that the main phase (80%) of Deccan eruptions

may have been very rapid, and would have ended at the Cretaceous-Palaeocene boundary

(Westerhold 2012, Fig.1). The long-term trend in benthic isotopes found by Westerhold et al

(2012) suggests that volcanic CO2 input is closely coupled to deep-sea warming. The slow

rise in temperature, after very-dramatic short-term cooling on the Cretaceous-Palaeocene

boundary, in the early Palaeocene may indicate increased input of CO2 associated with wide

scale eruptions and de-gassing from large igneous provinces such as Deccan.

5. Organic-rich sediments

There is a significant drop of up to 3‰ recorded in the δ13C carbon isotope values at the K-Pg

boundary (Figs.1 & 2). As alluded to in the previous section, this drop is mainly recorded in

planktonic samples and there is no corresponding drop in benthic isotopes (Frakes et al 1990)

further indicating, as reviewed in section 2, that the vertical carbon gradient in the early

Palaeocene was eradicated. The K-Pg extinction event is thought by most authors, to at least

some extent, to have caused a lowering in ocean productivity in the early Palaeocene. Hsu &

McKenzie (1985) suggest that an impact event, and the associated “dark” period due to clouds

of ejecta, brought about the decline of oceanic productivity and lead to a sterile dead ocean,

named the ‘Strangelove’ ocean. Continued studies by Arthur et al (1987) have observed this

low productivity to have lasted for as long as 1.5Myr, which they argue is a very long time for

the environment to be effected by a single impact event. This may support the argument of a

two stranded extinction as proposed by Alvarez et al (1984). During the early Palaeocene, a

decline in oceanic productivity would have caused an increase in atmospheric CO2 and

resulting in climate warming (Fig.1). This may have been compounded by volcanic eruptions

(Kellar 2005, Fig.1). A shift in δ18O isotopes from Cape Basin, South Atlantic (Miller et al

1987) and accompanying data from deep-sea profiles in the Pacific (Westerhold et al 2011,

Fig.1) indicate a short period of cooling at the K-Pg boundary (Fig.1) and then subsequent

steady warming during much of the early Palaeocene.

After a significant drop in δ13C at the K-Pg boundary (Figs. 1 & 2), δ13C isotopes rose steadily

to a large peak during the Late Palaeocene (Boersma et al 1979; Frakes et al 1990). δ13C

values in the late Palaeocene reached their highest levels for the entire Cenozoic (Charisi &

Schmitz 1994), with an increase in δ13C from around 1‰, in the early Palaeocene to

approximately 3.5‰ at around 60Ma (Shackleton 1986; Stott & Kennett 1989, 1990). This

maximum has been attributed to an amplified oxygen minimum zone, emanating from

enhanced biological productivity and associated with elevated organic-carbon accumulation

rates. This is supported by observations that both planktonic and benthich δ13C values

increased to the same extent, and thus indicating that organic carbon 12C was revoked from

the ocean system through burial as organic rich shale or coals. Studies linking δ13C, δ18O and

ocean temperature (Shackleton 1986; Stott & Kennet 1990, 1991; Westerhold et al 2011)

indicate that the oceans cooled considerably during high δ13C periods (early-late Palaeocene)

and warmed during low δ13C periods (early Palaeocene), (Fig.1).

Periodic slow excursions superimposed on the long-term trend of δ13C and δ18O isotopes

show swings of short-term 100kyr and 405kyr cycles (Charisi & Schmitz 1994; Westerhold et

al 2011) indicating the role of orbital forcing as the pace maker for paleoclimatic variability

on Milankovitch time scales. A dominant negative of both δ13C and δ18O isotopes, on the

100kyr period (Fig.1), characterizes the end of Palaeocene. This negative in the isotope record

coincides with the largest mass extinction event of deep-water benthic organisms during the

last 90Myr (Kennet & Stott 1991; Pak & Miller 1992). Earlier studies (Brass et al 1982)

emphasized the physical plausibility of deep-ocean circulation in the late Palaeocene to be

controlled by salinity, rather than temperature differences. However, as discussed in section 1,

a suggested rise of δ13C and δ18O in deep water sections, corresponding to deep-water oceanic

warming by as much of 6°C, indicate that major climatic and tectonic change is thought to

have brought about the decline of deep-water benthic organisms. Climatic change and

warming during the Late Palaeocene would also agree with reconstructed paleoclimate from

the Palaeocene-Eocene boundary, which experienced a large thermal maximum (PETM).

6. Land temperatures

The Cretaceous-Palaeocene transition was a period of vegetational change, characterised by

the appearance of the flowering plants and their increasing dominance in the vegetation

(Frakes et al 1990). In the early Palaeocene floral transition is suggested in some localities,

although the degree of this change is highly variable and the ability to resolve this change as

an alteration in global climate is yet to be achieved. A theorised impact event, if on land, is

thought to have caused large scale fallout of dust and ejecta that would have blocked out the

sun (Schulte et al 2001), and thus caused a decrease in photosynthesis and brought about

rapid climate cooling. A study by Wolbach et al (1998) suggests that soot particles found at

some localities at the K-Pg boundary were from large-scale forest fires that would have

destroyed vegetation. However, Cope and Chaloner (1980) suggest that such events were

common throughout the Cretaceous and that soot layers can be observed in earlier non-marine

sediments. As such, there does not appear to be any consistent effects of the hypothesised

meteroid impact on terrestial vegetation (Frakes et al 1990, and references therein). An

alternative stufy by Tschudy et al (1984) suggests that an increase in the percentage of fern

spores above the K-Pg boundary at several localities can be interpreted as a sequence of re-

colonization of vegetation in response to a dramatic event. These observations could implicate

an impact event, or could be equally attributed to a sudden onset of volcanic activity at the K-

Pg boundary as proposed by Keller (2005).

Wolfe & Upchurch (1986) suggest that during the early Pleoceene, in reaction to an impact

event, evergreen trees were more affected than their deciduous counterparts, which is implicit

of greater seasonal extremes during the Early Palaeocene. Results from terrestrial sediments

also suggest that the impact did not cause serious temperature changes on land but instead

caused a marked increase in precipitation. Rainforest vegetation became established in North

America in the early Palaeocene (Wolfe & Upchurch 1986). Global vegetation zones during

the Palaeocene seem to accommodate mostly rainforest type vegetation, with a wide

equatorial belt of tropical rainforest extending to latitudes of about 50° (Frakes et al 1990).

Paratropical, and broad leave evergreen vegetation were only accommodated in narrow bands

near to the poles. These vegetation zones remained largely similar for much of the

Palaeocene. Gradually warming oceanic temperatures, the opening up of sea gateways,

leading to greater atmospheric circulation and more rainfall, fuelled rainforest sustainability.

7. Conclusions

The early Palaeocene was characterised by a global environmental response to the K-Pg

extinction event. There is still debate as to whether extinction of cretaceous biotas occurred

suddenly, indicating an extraterrestrial impact, or whether extinctions occurred more

gradually, suggesting an alternative extinction mechanism. This perhaps highlights a common

ambiguity of reconstructing paleoenvironments, in that proxies are often of too lower

resolution, or have a locality spacing that is too far spread, to provide a constraint on the

environment over short timescales. However, proxies over longer time scales can provide a

constraint on environmental change within the limits of justifiable uncertainty. Oxygen and

carbon isotope records suggest a relatively cooler environment, at the K-Pg boundary, related

to the extinction event, with generally gradual increasing temperatures towards the Late

Palaeocene (Fig.1). Smaller transient warming events have been identified for the Palaeocene

epoch (Fig.1), but the exact character of these “events” is variable between different locations

and whether the warming events are truly reflective of global climate change is yet to

resolved (Westerhold et al 2011). Terrestrial environments adapted to initial changes at the K-

Pg boundary with a general increase in precipitation, and wide spread rainforests remained for

much of the Palaeocene. During the late Palaeocene, a shift in deep-ocean water sources from

cold to warm areas is suggested to have caused deep-ocean temperatures to rise. The

dominant decrease in δ13C and δ18O isotopes at this time coincides with the mass extinction of

deep-water benthic organisms. It is probable that the discussed changes in oceanic,

atmospheric and terrestrial environments were brought about by continued tectonic changes,

and were influenced to some extent, by the volcanic degassing of igneous provinces

throughout the Palaeocene.

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