stable isotope evidence for paleocene climate and environmental change

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EARTH SYSTEM & CLIMATE: PALEOCENE CLIMATE 2014

Stable isotope evidence for Paleocene climate and environmental change

CHARLIE KENZIE

Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE

Sent 28th April 2014

ABSTRACT

Deep marine carbonate data was collected near to the coast of Australia. Oxygen and carbon

isotopes were taken from the samples, and analyzed so to provide proxies on climate throughout the

Paleocene. Temperature corrected oxygen isotopes suggest that sea ice may have existed in the

early Paleocene, but that the vast majority of the Paleocene was ice-free. Temperatures are shown to

gradually increase throughout the Paleocene, and in contrast to previous studies, carbon isotopes are

shown to increase synonymously with temperature, with the exception of a cooling period in the

early-late Paleocene. This is attributed to the several phases of volcanic degassing from large

igneous provinces, which outstripped increased ocean productivity and resulted in rising CO2

levels. This suggests that volcanic degassing at this time was significant, and a major control on

Earth’s climate. Additionally, carbon isotopes are shown not to recover until approximately 1.5Ma

after a hypothesized bolide impact event at the K-Pg boundary. The long recovery time of ocean

productivity suggests that biotas across the Cretaceous-Tertiary boundary experienced a two-

stranded extinction, by which a more gradual extinction took place unrelated to an impact event at

the K-Pg boundary. Spectral analysis reveals periodicities in the data that cannot be attributed to

orbital forcing. However, the pattern indicates the significance of ‘feedback’ effects, which can

heavily influence short-term climate. One limitation is that the data is of too lower resolution to be

able to satisfactorily resolve short-term climate changes within the limits of justifiable uncertainty.

1. Introduction

The Paleocene epoch, which follows the upper cretaceous and precedes the Eocene, occupies a

timescale from approximately 66-55 Ma before present. The boundary between the Cretaceous and

the Palaeocene (K-Pg) is marked clearly in the fossil record, and is also palpable by anomalously



CHARLIE KENZIE

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. Although the

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 paper.

There has been much debate over the causes of the K-Pg extinction event, and paleoclimate at the

boundary has been well studied. However, to my knowledge, more detailed investigations of

Earth’s environment during the whole of the Paleocene are less documented. This short paper

intends to investigate whether the effects of the extinction event can be observed in the early

Paleocene, and thus giving clues as to the nature of the extinction since longer-term effects would

count against the theory of a bolide impact. Marine carbonate data from Australia will be used to

infer paleoclimate in more detail for the whole of the Paleocene, and the results will be compared to

exisiting scientific literature.

2. Methods

Isotopic compositions have long been used to infer past climate. Oxygen isotopic compositions of

marine carbonates can be used to infer temperature due to the temperature dependent fractionation

of oxygen isotopes between seawater and calcite. Changes in !18O can be used to indicate when the

world was glaciated by assuming, that when bottom waters are cold and frigid, this is sufficient to

support large ice sheets (Miller et al. 1987). !18O can be corrected for Paleotemperature utilizing:



CHARLIE KENZIE

T °C ( ) = 16.9 " 4.2 # c "#

w( ) +0.13 # c "#

w( )2

where T is the paleotemperature, !w is the global seawater composition, and !c is the measured value

in calcite (Anderson 1983). Since previous studies have shown that the extent of ice in the

Paleocene was limited, an assumed value of !w = -1.2‰ is reasonable (Tripati et al. 2001; Miller et

al. 1987). Deep marine carbonate data was collected off the coast of Australia and corrected to infer

paleotemperature (Fig. 1b). Carbon isotope data was also taken from the same marine sample, and

can be used to indicate productivity and anoxia of oceans, and thus to some extent, the amount of

CO2 in the atmosphere. Subsequently, both of these isotopes can be used as sensitive indicators of

global climate.

3. Results

3.1 Carbon Isotope Trend

The overall trend of the marine data shows an increase in !13C through the early Paleocene, with a

subsequent slight fall in the middle Paleocene between 62-60Ma, and then, a continued increase

through to the late Paleocene. The general trend is easily visible on a 3-point moving average plot

(Fig.2a). The overall trend is sequentially broken by a number of smaller peaks and troughs, more

dramatically throughout the middle Paleocene, and to a lesser extent in the late Paleocene. To

investigate the periodicity of the data, a simple spectral analysis tool was utilized (Fig.3b). The

computation resulted in a main positive peak corresponding to around 5Ma, and a second, smaller

peak, corresponding to a periodicity of around 2.5 Ma.

3.2 Oxygen Isotope Trend

The overall trend shows an initial decrease in !18O in the early Paleocene, succeeded by a flattening

of oxygen isotopes in the middle Palecoene, followed by a slight increase in the early-late

Paleocene, and a final dramatic decrease in the late-late Paleoceene. A 3-point moving average

shows this general trend in Fig.2b. The temperature corrected oxygen isotope data displays the

same trend, but inversed, an obvious outcome when assuming warmer temperatures correspond to a



CHARLIE KENZIE

higher ratio of18

O left in water. The temperature corrections are first computed assuming an ice

free world during the Paleocene T1, employing a value of !w = -1.2‰. To compare the results with

an upper boundary, a second temperature computation T2 is carried out using a value of

!w = -0.28‰, which corresponds to today’s amount of sea ice (Miller et al. 1987). The trend is the

same, as would be expected, but the calculated temperatures are warmer using the modern seawater

ratio. Since, average seawater temperatures below 2°C could result in waters frigid enough to

maintain sea ice, a line at 2°C is drawn across the graph, and shows that ice sheets may have existed

in the early Paleocene when assuming an ice free value of !w. Simple spectral analysis was also

computed for the oxygen data, and the results show a similar single large peak corresponding to a

periodicity of around 6 Ma, and a second peak corresponding to a periodicity of around 1.8 Ma

(Fig.3a). Spectral analysis of both data sets, reveal similar dominant periodicities in both, and

possibly indicating the mutual relationship of the two proxies. This is further highlighted when

eyeballing Fig.1a, and more quantitatively, when the two isotope proxies are plotted against each

other (Fig.2c). The plot calculates a correlation coefficient of -0.5386, which when compared using

a table of pearson product-moment correlation coefficients, lies within a 0.02 significance level, or

98% significance.

4. Discussion

Previous studies have recorded a significant drop in !13

C isotopes at the K-Pg boundary, which goes

some way to explain the low !13C levels observed in the Australian marine data in the early

Paleocene. The causes of the extinction are debated, but the most accepted theory links a bolide

impact at around 66 Ma to the decline. Despite the debate, the effect of the K-Pg extinction event is

thought by most, to at least some extent, to have caused a lowering in ocean productivity in the

early Paleocene, and to have brought about a sterile dead ocean named the ‘Strangelove’ ocean

(Hsu & McKenzie 1985). The marine carbonates from Australia show a small trough at around 65

Ma, corresponding to the sterile conditions caused by extinction, and furthermore, highlight that



CHARLIE KENZIE

!13

C levels did not start to recover until after 65-65.5 Ma, 1 – 1.5 Ma after the proposed impact

event at the K-Pg boundary. These observations, which agree with previous studies (Arthur et al.

1987), suggest that low productivity lasted for as long as 1.5 Ma, which is a very long time for the

environment to be effected by a single impact event. This may have important implications about

the mechanisms behind extinction, and the length of recovery indicated by the !13C isotope data,

may support a two-stranded extinction, by which a more gradual extinction took place unrelated to

an impact event at the K-Pg boundary.

The Strangelove conditions after the K-Pg extinction event resulted in sluggish oceanic and

atmospheric circulation, leading to slightly cooler temperatures in the early Paleocene. Previous

studies have revealed very little evidence for permanent ice sheets. However, temperature corrected

!18O, with a seawater composition of !w = -1.2‰, shows that at very short intervals during the early

Paleocene, it may have been possible to sustain ice sheets. Furthermore, Paleocene aged clasts of

fine-shales were found on Spisbergen (Arctic Norway), and interpreted as ice-raft deposits (Dallan

1977), supporting the possibility of sea-ice in the early Paleocene. However, if a seawater

composition comparable to a modern day value is used (!w = -0.4‰), then the temperature corrected

data indicates that ice could not be sustained at any time during the Paleocene, and considering the

limited resolution of the isotope data, the presence of sea-ice during the early Paleocene remains

ambiguous. Despite this, the temperature corrected !18

O data indicates significant enough climate

warming to indicate that no ice sheets existed throughout the majority of the Paleocene.

After the significantly low levels of carbon isotopes in the early Paleocene, !13C rises steadily

throughout much of the remainder of the Paleocene to a large peak (3.5‰) in the late Paleocene;

some of the highest levels observed for the whole of the Cenozoic (Charisi & Schimtz 1994;

Shackleton 1986; Stott & Kennett 1989, 1990). The steady increase is attributed to an amplified

oxygen minimum zone, emanating from enhanced biological productivity and associated with



CHARLIE KENZIE

elevated organic-carbon accumulation rates. These observations are supported by previous studies,

which suggest that both planktonic and benthic !13C values increased to the same extent, and thus

indicating that organic 12C was revoked from the ocean system through burial as organic rich shale

or coals.

This study, amongst others (Shackleton 1986; Stott & Kennet 1990, 1991; Westerhold et al. 2011),

indicates that oceans may have cooled during high !13C periods, for example during the early-late

Paleocene (59.5-57.5 Ma). This agrees with a generally accepted correlation, that an increase in

!13

C isotopes suggests a cooler climate, since more productive oceans would lead to less

atmospheric CO2, and thus, cooler temperatures. However, in contrast to previous work, the marine

data in this study show a strong negative correlation between oxygen and carbon isotopes, and with

the exception of the cool period in the early-late Paleocene, show that !13C increases synonymously

with temperature. I propose that this anomaly is caused by the significant effects of various stages

of volcanic de-gassing from large igneous provinces (LIPs). Eruptions were extensive enough such

that any decrease in atmospheric CO2, caused by increasing ocean productivity, was outstripped by

the addition of CO2 from de-gassing. Extended periods of de-gassing from LIPs are shown on

Fig.1b. It is clear that warming occurred over periods that either directly followed, or were

synonymous with, major eruptive phases. Since the early-late Paleocene is not synchronised with

any eruptive phase, it is plausible that the effects of volcanic CO2 were negligible at this time, such

that increasing ocean productivity (shown by a continued increase in !13C) could now cause a fall in

atmospheric CO2 over this period, and thus leading to a fall in temperature. Towards the late-late

Paleocene however, another LIP eruptive phase began, and the addition of CO 2 outstripped that

which could be absorbed by the oceans, causing the temperature to rise. This has important

implications to the sensitivity of Earth’s climate to volcanic eruptions, and suggests a clear

mechanism behind the continued warming of the planet towards the late Paleocene and early

Eocene, despite steadily rising ocean productivity indicated by increased !13C isotopes.



CHARLIE KENZIE

The increase in temperature in the late Paleocene, is linked to other factors than just volcanic

degassing. During the late Paleoceene, hydrothermal activity introduced nutrient rich anoxic water,

and tectonic reorganisation increased ocean mixing (Vogt, 1979) (Fig.4). This resulted in warm

saline and oxygen-deficient deep water to mix with cold, nutrient-depleted waters at higher

latitudes, thus shifting the locus of ongoing deep-water formation from cold to warm waters, and

causing deep ocean water temperatures to rise dramatically. The lack of cooled near surface waters

emanating from polar regions allowed the penetration of warmed boundary currents to higher

latitudes (Boersma & Permoli-Silva 1983), and is likely to have contributed to a concurrent

warming temperature. These observations also agree with studies that suggest that a wide equatorial

belt of tropical rainforest extended to latitudes of about 50° throughout the late Paleocene (Frakes

et al. 1990), brought about by gradually warming oceanic temperatures and the opening of sea

gateways, which led to greater atmospheric circulation and more rainfall, fuelling rainforest

sustainability. Increased terrestrial rainfall towards the end of the Paleocene may also have

increased eutrophication and explain the continued positive !13C excursion in the late Paleocene,

and goes some way to explain the demise of the Paleocene deep-sea fauna, a commonly measured

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

Spectral analysis reveals a dominant 5-6 Ma periodicity in the !13

C and !18

O isotopes. Periodic

slow excursions superimposed on the long-term trend of the isotopes, show swings of around 2 Ma.

This is in contrast to previous studies, which have found superimposed periods of short-term 100kyr

and 405kyr cycles (Charisi & Schmitz 1994; Westerhold et al. 2011), clearly indicating orbital

forcing. The absence of orbital forcing in this study is explained by the comparably low resolution

of the data, making it impossible to determine short-term time periods. To this end, it is equally

difficult to attribute the sudden changes in the data to smaller hypothermals or more subtle changes

in environment causing differing isotope ratios. However, the sawtooth pattern of the data does



CHARLIE KENZIE

indicate the presence of non-linear “feedback effects”, such as vegetation growth, mountain

building or changes in ocean circulation, which have been found in many studies as an important

control on paleoclimate. 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 environment over short timescales. However, proxies

used over longer time scales, as has been done in this study, can provide constraints on

environmental change within the limits of justifiable uncertainty.

One further problem with reconstructing past climate is the possibility of non-linear interplay

between different limitation factors of the observably proxy information, and the possibility of non-

stationary proxy-climate regimes, which give rise to ambiguity and many uncertainties (Bothe &

Zanchettin 2013). To combat this, in this study !13C data is compared with !

18O data. On the other

hand, the assumption that they are mutually related may cause further complication and ambiguity.

In spite of this, several other studies have linked the combined use of these proxies to paleoclimate,

and the correlation found between the two proxies in this study is significant (within 98%

signifcance). Another complication with using marine carbonate data is that one has to assume that

post-depositional diagnetic recrystallisation has not changed the carbonate’s original isotopic

composition. Quantitative corrections for diagenisis require analytical procedures out of the limits

of this paper. Nevertheless, it has been found that carbonates precipitated in high-latitude oceans,

such as those near Australia, experienced very little rapid re-crystallisation due to diagenisis

(Schrag et al. 1995).

5. Conclusions

Carbon isotopes in the early Paleocene are not shown to recover until approximately 1.5Ma after a

hypothesized bolide impact event at the K-Pg boundary, which seems a long time for oceans to

recover after a single event, and suggests that biotas across the Cretaceous-Tertiary boundary



CHARLIE KENZIE

experienced a two-stranded extinction, by which a more gradual extinction took place unrelated to

an impact event at the K-Pg boundary. Temperature corrected oxygen isotopes suggest that sea ice

may have existed in the early Paleocene, however this remains ambiguous. Nonetheless, the data

show that temperatures would have been much too high for ice to be sustained for the vast majority

of the Paleocene. Temperature increases throughout the Paleocene seem to coincide with eruptive

episodes from large igneous provinces. A short cooling period in the early-late Paleocene is not

synchronous with an eruptive phase, and increasing ocean productivity caused a fall in atmospheric

CO2 over this period, thus leading to a fall in temperature. This suggests that volcanic degassing at

this time was significant, and a major control on Earth’s climate. Generally increasing temperatures

towards the late Paleocene coincide with changing ocean-water sources brought about by increased

ocean mixing, hydrothermal activity and ocean anoxia. Spectral analysis reveals periodicities in the

data that cannot be attributed to orbital forcing. However, the pattern indicates the significance of

‘feedback’ effects, which can heavily influence short-term climate. One limitation is that the data is

of too lower resolution to be able to satisfactorily resolve short-term climate changes within the

limits of justifiable uncertainty.

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to sedimentologic and paleoenvironmental problems." University of Illinois, 1983.

BOTHE, O., and D. Zanchettin. "Ambiguity of large scale temperature reconstructions from

artificial tree growth in millennial climate simulations." arxiv.org. 2010.

http://arxiv.org/ftp/arxiv/papers/1207/1207.2279.pdf (accessed January 2014).

CHARISI, S. D., and B. Schmitz. "Stable d13C,d180) and strontium (87Sr/86Sr)isotopes through

the Paleocene at Gebel Aweina, eastern Tethyan region ."

Palaeogeography,Palaeoclimatology,Palaeoecology (Elsevier) 116 (1994): 193-129.

HAY, W.W, DeConto, R., Wold, C.N., Wilson, K.M., Voigt, S., Schulz, M., Wold-Rossby, A.,

Dullo, W.-C., Ronov, A.B., Balukhovsky, A.N. and E. Soeding (1999): ALTERNATIVE

GLOBAL CRETACEOUS PALEOGEOGRAPHY, in Barrera, E. and Johnson, C. (eds.), The



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Evolution of Cretaceous Ocean/Climate Systems, Geological Society of America Special

Paper 332, pp. 1-47.

KENNETT, J. P., and L. D. Stott. "Abrupt deep-sea warming, palaeoceanographic changes and

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225-229.

KENNETT, J. P., and L. D. Stott. "PROTEUS AND PROTO-OCEANUS: ANCESTRAL

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MILLER, K. G., T. R. Janecek, M. E. Katz, and D. J. Keil. "ABYSSAL CIRCULATION AND

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(Elsevier Science), 1995: 2265-2278.

TRIPATI, A., J. Zachos, L. Jr. Marincovich, and K. Bice. "Late Paleocene Arctic coastal climate

inferred from molluscan stable and radiogenic isotope ratios ." Palaeo., 2001: 101-113.

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Possibly Ice Free

Possible Ice Sheets

Figure1a Deep marine carbonate oxygen (solid line) and

carbon (dashed line) isotopes. Show a broadly negative

correlation. Note how d13C does not start recver untilapproximately 65Ma, 1.5Ma after a hypothesized bolide

impact event. Figure 1b Temperature corrected oxygen

isotope data. T1 (solid line) shows temperature corrected

assuming a seawater composition of an ice-free globe. T2

(dashed line) uses a present day seawater composition.

Dotted line shows the possibly boundary for ice to be

sustained. Only plausible for the very early Paleocene.

Figure 2a-2b 3-point moving

averages of carbon and oxygen data

respectively, showing general trends

over time. Figure 2c cross-plot of

oxygen and carbon isotope data,

showing a strong negative

correlation between the two.

a

b

a

b

c

Figure 3a Spectral analysis of oxygen isotope data, showing a dominant

frequency corresponding to a period of around 6Ma, and a smaller peak

corresponding to 1.8 Ma. Figure 3b Spectral analysis of carbon isotope

data, showing a similar single dominant peak corresponding to a period

of around 5 Ma.

Figure 4 Paleographic map of the

Early Paleocene 65Ma before

present (Hay et al. 1999).

stable isotope evidence for paleocene climate and environmental change

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