A Comparative Study of Accumulation Rates Derived by Th and He Isotope Analysis of Marine Sediments

by Franco Marcantonio, Niraj Kumar, Martin Stute, Robert F. Anderson, Michele A. Seidl, Peter Schlosser

published in Earth and Planetary Science Letters , volume 133 , pp. 549-555.


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We present a detailed down-core analysis of helium isotope ratios and concentrations for bulk sediments from the central Equatorial Pacific that span the last two glacial-interglacial cycles. Measured 3 He/ 4 He ratios range from 1.0x10 -5 to 2.1x 10 -4 , or 7.4 to 149 times the atmospheric ratio. The 3 He from interplanetary dust particles (IDPs) constitutes virtually all of the 3 He measured within the sediment. Because carbonate accumulation rates are high in the Equatorial Pacific, the measured 3 He concentrations are lower than have been measured elsewhere, and range from 4.7x10 -13 to 3.0x10 -12 cm 3 STP . g -1 .

If the cosmic dust 3 He-flux is constant with time, sediment mass accumulation rates can be determined from the 3 He concentration in sediments. The excess 230 Th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in-situ decay of 234 U in sea water. To first order, initial excess 230 Th activities correlate with 3 He concentrations within this core. Based on the 230 Th results, we estimate the 3 He-flux to the Earth's surface as 9.6+/-2.0 cm 3 STP . cm -2. a -1 . If this flux has remained constant over extended periods of time, it can be used to determine sediment accumulation rates beyond the 230 Th range (300,000 years).


Helium isotope analyses of equatorial Pacific sediment samples from a single core can be explained by mixing between IDP and terrigenous components. Biogenic phases contain essentially no helium and act as a dilutant to the helium concentration signal. Virtually all of the 3 He in the sediment is derived from the IDP component.

To first order, over the past 200 ka, 3 He concentration variations correlate well with 230 Thxs o activity variations which suggests that the flux of extraterrestrial 3 He has remained relatively constant. By assuming a constant flux of 230 Th, we calculate an extraterrestrial 3 He flux of about 9.6x10 -16 cm 3 STP . cm -2. a -1 , variable to within 20%. This number agrees with previous estimates based on entirely different methods of calculation [9, 21, 22] . The variability in 230 Thxs o / 3 He ratio indicates changes in either the flux of 230 Th, 3 He, or both.

Unlike the Th method which is limited to the past 300 ka, normalizing to a constant flux of 3 He may expand our ability to determine instantaneous sediment mass accumulation rates and the corresponding fluxes of different sedimentary components on much longer timescales.


Figure Captions


Figure 1a. 3 He concentration versus 3 He/ 4 He ratio. Data from the literature [6, 9] and this study.



1b. Same as Figure 1a except that data from this study has been normalized to the % terrigenous fraction. Agreement with previous pelagic data is observed.


Figure 2. 3 He concentration versus 3 He/ 4 He ratio. The data approximate mixing between two components, an IDP component ( 3 He~3x10 -5 cm 3 STP . g -1 , 3 He/ 4 He=2.4x 10 -4 ; [15] ) and a terrigenous component ( 3 He~3x10 -15 cm 3 STP . g -1 , 3 He/ 4 He=1x 10 -8 ; [3] )



Figure 3. 230 Thxs o activity and 3 He concentration versus age (from 18 O stratigraphy,

[11] ). Glacial-interglacial timescale (bar at bottom of figure) is drawn for reference.



Figure 4. 230 Thxs o activity versus 3 He concentration for equatorial Pacific sediment samples. The best fit line excludes the two bracketed points (explained in text). The correlation coefficient, R 2 , is 0.70. The positive intercept, 3.5 dpm . g -1 , is within 2[[sigma]] uncertainty of zero.