An alternative age model for the Paleocene–Eocene thermal maximum using extraterrestrial 3He

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Abstract

A continuous age model for the brief climate excursion at the Paleocene–Eocene boundary has been constructed by assuming a constant flux of extraterrestrial 3He (3HeET) to the seafloor. 3HeET measurements from ODP Site 690 provide quantitative evidence for the rapid onset (<few kyr) and short duration (<120 kyr) of global warming and of the associated disturbance to the Earth’s surficial carbon budget at this time. These observations support astronomically calibrated age models indicating extremely rapid release of isotopically light carbon, possibly from seafloor methane hydrate, as the proximal cause of the event. However, the 3HeET technique indicates a previously unrecognized and extreme increase in sedimentation rate coincident with the return of climate proxies to pre-event values. The 3HeET-based age model thus suggests a far more rapid recovery from the climatic perturbation than previously proposed or predicted on the basis of the modern carbon cycle, and so may indicate additional or accelerated mechanisms of carbon removal from the ocean–atmosphere system during this period. 3HeET was also measured at ODP Site 1051 to test the validity of the Site 690 chronology. Comparison of these data sets seems to require removal of several tens of kyr of sediment within the climatic excursion at Site 1051, an observation consistent with sediment structures and previous age modeling efforts. The Site 1051 age model shows a ∼30 kyr period in which climate proxies return toward pre-event values, after which they remain invariant for ∼80 kyr. If this rise represents the recovery interval identified at Site 690, then the 3HeET-based age models of the two sites are in good agreement. However, alternative interpretations are possible, and work on less disrupted sites is required to evaluate the reliability of the proposed new chronology of the climate excursion. Regardless of these details, this work shows that the 3HeET technique can provide useful independent evidence for the development and testing of astronomically calibrated age models.

Introduction

The Paleocene/Eocene Thermal Maximum (PETM) was a brief global warming event at ∼55 Ma embedded within the long-term warming trend of the early Cenozoic [1], [2], [3]. It is among the best known examples of a transient climate perturbation, with possible relevance to understanding humans’ impact on the climate system [4]. A negative excursion in the δ18O of foraminifera at this time indicates warming of high latitude surface and deep water temperatures by ∼6°C [1], and is accompanied by extinction and radiation of both marine and terrestrial species [5], [6], [7], and by mineralogical evidence for changes in the global hydrologic cycle [8], [9]. A critical aspect of the PETM is the coincident excursion in foraminiferal δ13C by −2 to −3‰, presumably from injection of isotopically light carbon into the ocean–atmosphere system ([1], Fig. 1).

The key to interpreting the cause of this event is its apparent rapidity [4], [10], [11]. The first observations of a fast onset and 100–200 kyr duration of the PETM were based on the assumption of a constant sedimentation rate between dated magnetostratigraphic boundaries [1], but this assumption is questionable given the extreme changes in sediment composition coincident with the event at many localities (e.g., [12]). Nevertheless, recent astronomically calibrated age models, based on presumed precessional cycles observed in sediment properties, support the rapid pace of the climate excursion [13], [14], [15]. Thermally or mechanically induced decomposition of seafloor methane hydrate is the preferred source of the carbon, primarily because it appears to be the only isotopically appropriate source that can be released at the very rapid inferred rate [4], [10]. Thus the interpretation of the PETM rests in large part on the validity of the age models. In addition the input history and fate of the isotopically light carbon and the overall response and recovery of the Earth’s climate system to the perturbation have been inferred by reference to the astronomical age models [13], [14], [16], [17].

To be made mutually consistent the astronomical age models for the PETM at several sites require arbitrary assumptions that are not easily tested. Based on magnetic susceptibility and Fe concentration measurements at Ocean Drilling Program (ODP) Site 1051 Norris and Röhl [13] identified seven precessional cycles within the carbon isotope excursion (CIE) and concluded that the event lasted ∼150 kyr (Fig. 1). However, using Fe and Ca data at the relatively expanded ODP Site 690 Röhl et al. [14] identified 11 cycles within the CIE and proposed a substantially longer duration for the PETM. This discrepancy was attributed to incorrect identification of precessional peaks by Norris and Röhl [13], and alternative identifications were made in the original Site 1051 data using Site 690 as a template. To match the 690 and 1051 records Röhl et al. [14] further proposed that about 50 kyr of section was removed from Site 1051 by sediment disruption observed near the base of the isotopic excursion. The result of these adjustments is a reasonable agreement between the carbon isotopic profiles at these two sites (Fig. 1).

This reinterpretation underscores that identification of presumed astronomically forced peaks in noisy geochemical or geophysical data, and the resulting age models, are subjective and potentially prone to unquantifiable errors. This is especially true during intervals of rapid sedimentological change like the PETM. In addition, the use of Site 690 as a template against which to match other cores places substantial importance on this age model. These issues coupled with the significance of the distribution of time for understanding the PETM motivated us to establish an independent and quantitative age model for comparison with the astronomical models.

Here we estimate the pace of the PETM using extraterrestrial 3He as a constant flux proxy [18], [19], [20]. 3He in deep sea sediments is derived mostly from small interplanetary dust particles (IDPs) [21], [22]. Terrestrial 3He produced by several nuclear processes is also carried to the seafloor in minor amounts by detrital minerals [21], [22], [23]. A potential additional source, volcanic ash, may be erupted with a small amount of mantle-derived 3He, but this source is inconsequential to the 3He budget in typical deep sea sediments [23]. Because terrestrial-detrital and extraterrestrial helium have very different 3He/4He ratios, isotopic measurements can be used to establish the concentration of extraterrestrial 3He (3HeET) in a sample [21]. For a given 3HeET flux (F, in atoms/area/time), 3HeET is related to the sediment mass accumulation rate (α) by dilution: 3HeET=F/α [18]. This relationship can be inverted to calculate mass accumulation rates from 3HeET measurements, provided F can be determined, for example by measuring the amount of 3HeET accumulated over a known time interval. F calculated in this way is termed the apparent 3He flux, and includes the effects of diagenetic or diffusive He loss and possible sedimentary focusing of IDPs [23]. The use of extraterrestrial tracers such as Ir and 3He for determination of sedimentation rate is not a new idea [18], [24] but has not yet been widely applied. Marcantonio and coworkers [19], [20] compared 3He-based mass accumulation rates with those derived from excess 230Th in Quaternary sediments, with positive results, and the technique was recently used to estimate the depositional interval of the Cretaceous/Tertiary boundary clay [25].

Unlike astronomical calibration, which is limited to a temporal resolution of about one precessional cycle (∼20 kyr), the 3He method provides instantaneous sedimentation rate estimates for every analyzed sample, from which a complete and quantitative age model can be established. Many PETM sections (e.g., [5], [26]) appear to be condensed by carbonate dissolution because the event is associated with shallowing of the lysocline and CCD [4]. Condensed section can be difficult or impossible to quantify with cyclostratigraphy and complicates interpretation of the PETM at some sites (e.g., [27]), but is readily identified by elevated extraterrestrial 3He concentrations [25]. Similarly the 3He method can reveal very large or rapid changes in sedimentation rate which can confound the cyclostratigraphic approach. However, the 3He method shares with cyclostratigraphy the inability to reveal that section is entirely missing.

Temporal variability of the apparent 3He flux is the largest source of uncertainty in this method. Over millions of years the apparent flux varies by about an order of magnitude [23], but over intervals comparable to the proposed PETM duration the variability is probably much smaller [19], [20], [28]. This variability may arise from temporal changes in the delivery of 3He from space related to orbital dynamics or solar system events [23], or from site-specific sedimentary phenomena such as focusing or diagenesis [19], [20], [23], [28], [29]. Uncertainties in the apparent flux can be reduced by calibration using a sedimentary section located close in space and time to the section of interest. Similarly, comparisons among multiple sites provide an empirical test of the resulting age models, because sedimentary phenomena affecting the apparent flux are unlikely to be identical in timing and magnitude at geographically separated localities.

Section snippets

Samples and methods

We analyzed samples from ODP Site 690, located on Maud Rise in the Weddell Sea (65°16′S, 01°20′E [12]) and from Site 1051 located on Blake Nose in the western North Atlantic ocean (30°03′N, 76°21′E [30]). Hole 690B contains the most complete and detailed PETM record yet described [1], [14], [16], [17], and is the primary focus of the present study. A total of 158 samples were analyzed over a 7 m interval including the PETM. For the ∼70 cm including the CIE sampling was continuous at 1 cm

Site 690

The results from Site 690 are shown in Fig. 2 (and Appendix A in the Background Data Set1). 3He/4He ratios range from ∼1 to ∼11×10−7 excluding a single outlier at 23×10−7. While individual points scatter, there is no discernible trend in the ratio data (Fig. 2B). These 3He/4He ratios dictate that 3He is >88% extraterrestrial for most samples, with a range from 62% to 99%. In all samples 4He is derived almost completely from terrestrial detrital minerals. In

Sedimentation rates

3HeET-based sedimentation rates at Site 690 are in good agreement with the astronomically calibrated model [14] up to 170 mbsf, suggesting that this method provides a reliable estimate of rates through the early part of the PETM. The 3HeET method adds temporal resolution, but does not change the established picture. Of particular interest is that the 3He model shows no evidence of extremely low sedimentation rates (i.e., highly condensed section) that might have been missed by cycle counting.

Conclusions

The assumption of a constant apparent flux of extraterrestrial 3He to the seafloor leads to a continuous age model for the PETM at Site 690 that is an alternative to existing age models [1], [14]. The 3HeET model confirms the rapidity of the initial decline in δ13C values at the base of the CIE (<few kyr) and the short duration of the entire event (∼120 kyr); there is no evidence for very condensed section that might cause the cyclostratigraphic approach [14] to significantly underestimate

Supplementary data

. Appendix 1: Isotopic Data and Sedimentation Rate ModelAppendix 2: Extraterrestrial 3He Flux Estimates.

Acknowledgements

We thank Tim Bralower for guidance, encouragement and helpful comments on the manuscript, Debbie Thomas for preparation of samples from Site 690, and constructive reviews by Jim Zachos and Franco Marcantonio. This work was supported by NSF EAR-9909448 and by a Fellowship from the David and Lucille Packard Foundation to K.A.F.[BARD]

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