An official website of the United States government
Abstract We report40Ar‐39Ar step‐heating ages of Paleocene‐Eocene (P‐E) boundary impact spherules from Atlantic Margin coastal plain and open ocean sites. We test the hypothesis that the P‐E spherules are reworked from an earlier event (e.g., K‐Pg impact at ~66 Ma), which predicts a cooling age discordant from their depositional age of 55.93 ± 0.05 Ma at the P‐E boundary. Isochrons from the step‐heating analysis yield40Ar‐36Ar intercepts in excess of the modern in most cases, indicating that the spherules have excess radiogenic Ar (40Ar*), typical of impact glasses incompletely degassed before solidification. The weighted mean of the isochron‐corrected plateau age is 54.2 ± 2.5 Ma (1σ), and their isochron age is 55.4 ± 4.0 Ma, both indistinguishable from their P‐E depositional age, not supporting the K‐Pg reworking hypothesis. This is consistent with all other stratigraphic and geochemical evidence for an impact at the P‐E boundary and ejecta distribution by air fall.
more »
« less
The Ocean's Global 39 Ar Distribution Estimated With an Ocean Circulation Inverse Model
Abstract 39Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of39Ar using a steady ocean circulation inverse model. Our estimates match available39Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that39Ar traces out the world ocean's ventilation pathways and that the39Ar age ΓArand the ideal mean age have broadly similar large‐scale patterns. At the surface,39Ar is close to saturated except at high latitudes. Undersaturation imparts a finite39Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating ΓArby ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of39Ar, thus providing guidance for future measurements.
more »
« less
- Award ID(s):
- 1658392
- PAR ID:
- 10360180
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 13
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 7491-7499
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Improvements in analytical procedures in parallel with intercalibration of40Ar/39Ar and U–Pb methods and laboratories, spurred since 2003 by the EarthTime geochronology community initiative, have led to ±2σuncertainties of the order of 50–100 ka, or better, for Cretaceous ash beds. Assembled here are 5740Ar/39Ar ages and 17238U–206Pb ages of volcanic ash beds in strata of the Western Interior Basin of North America determined during the last 15 years since these improvements have been made. These age determinations span from 108 Ma in the middle Albian to 66 Ma in the latest Maastrichtian. Five of the40Ar/39Ar ages from Campanian and Maastrichtian strata are newly reported here, whereas the remainder are from the literature. Building on the pioneering work of John Obradovich and Bill Cobban, where possible these age determinations are tied to ammonite and inoceramid biostratigraphy. For most ash beds, the temporal uncertainties, unlike earlier timescales for the Western Interior Basin, are much shorter than the duration of fossil biozones. Proposed ages for stage boundaries based on this review of the radioisotopic ages include: Maastrichtian–Danian, 66.02 ± 0.08 Ma; Campanian–Maastrichtian, 72.20 ± 0.20 Ma; Santonian–Campanian, 84.19 ± 0.38 Ma; Coniacian–Santonian, 86.49 ± 0.44 Ma; Turonian–Coniacian, 89.75 ± 0.38 Ma; Cenomanian–Turonian, 93.95 ± 0.05 Ma; Albian–Cenomanian, 100.00 ± 0.40 Ma. Six bentonites that occur within theVascoceras diartianum, Neocardiocerus juddi, Prionocylus macombi, Scaphites preventricosus, Scaphites depressusandDesmoscaphites bassleriammonite zones, dated using both40Ar/39Ar and U–Pb methods, yield ages in agreement to within 150 ka and form the backbone of the Western Interior Basin timescale. In parallel, improvements in the taxonomy of ammonites, inoceramids and foraminifera, and recent field work, are better establishing the biostratigraphic framework for these age determinations. Each of these efforts contributes to the progressive refinement of the chronostratigraphic framework of the Western Interior Basin, and enhances its utility for global correlation.more » « less
-
Radiocarbon dating is arguably the most common method for dating Quaternary deposits. However, accurate age assignments using radiocarbon dating are dependent on knowing the radiocarbon reservoir. For the coastal waters across Antarctica, the radiocarbon reservoirs show significant variation, ranging from 700 to 6000 years depending on the material dated and the period in question. In this study, we examine the radiocarbon reservoir age for the shallow waters of the Southern Ocean using 23 whale bones salvaged from commercial whaling operations on or near the Western Antarctic Peninsula between 1904 and 1916. The species origin of the bones had been identified previously as humpback, fin, or blue whales using sequences of mitochondrial (mt)DNA. We find an average reservoir age of 1050 ± 135 years for these 23 whale bones, with a <100-year difference in the reservoir age by species. A comparison between our results and other studies through the Holocene suggest that the Southern Ocean surface water radiocarbon reservoir age is of a similar magnitude across much of Antarctica and has not significantly changed for the last 14,000 years. Combining our new ages with existing data sets provides insight to the stability of the Southern Ocean marine radiocarbon reservoir age, enhancing our understanding of ocean ventilation and upwelling dynamics throughout the Holocene.more » « less
-
Paired measurements of14C/12C and230Th ages from two Hulu Cave stalagmites complete a precise record of atmospheric14C covering the full range of the14C dating method (~54,000 years). Over the last glacial period, atmospheric14C/12C ranges from values similar to modern values to values 1.70 times higher (42,000 to 39,000 years ago). The latter correspond to14C ages 5200 years less than calibrated ages and correlate with the Laschamp geomagnetic excursion followed by Heinrich Stadial 4. Millennial-scale variations are largely attributable to Earth’s magnetic field changes and in part to climate-related changes in the oceanic carbon cycle. A progressive shift to lower14C/12C values between 25,000 and 11,000 years ago is likely related, in part, to progressively increasing ocean ventilation rates.more » « less
-
Mattoon, C.M.; Vogt, R.; Escher, J.; Thompson, I. (Ed.)The cross-section of the thermal neutron capture41Ar(n,γ)42Ar(t1/2=32.9 y) reaction was measured by irradiating a40Ar sample at the high-flux reactor of Institut Laue-Langevin (ILL) Grenoble, France. The signature of the two-neutron capture has been observed by measuring the growth curve and identifying the 1524.6 keV γ-lines of the shorter-lived42K(12.4 h) β−daughter of42Ar. Our preliminary value of the41Ar(n,γ)42Ar thermal cross section is 240(80) mb at 25.3 meV. For the first time, direct counting of42Ar was performed using the ultra-high sensitivity technique of noble gas accelerator mass spectrometry (NOGAMS) at Argonne National Laboratory, USA.more » « less
