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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Abstract

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 3He/4He ratios range from 1.0x10-5 to 2.1x 10-4, or 7.4 to 149 times the atmospheric ratio. The 3He from interplanetary dust particles (IDPs) constitutes virtually all of the 3He measured within the sediment. Because carbonate accumulation rates are high in the Equatorial Pacific, the measured 3He concentrations are lower than have been measured elsewhere, and range from 4.7x10-13 to 3.0x10-12 cm3STP.g-1.

If the cosmic dust 3He-flux is constant with time, sediment mass accumulation rates can be determined from the 3He concentration in sediments. The excess 230Th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in-situ decay of 234U in sea water. To first order, initial excess 230Th activities correlate with 3He concentrations within this core. Based on the 230Th results, we estimate the 3He-flux to the Earth's surface as 9.6+/-2.0 cm3STP.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 230Th range (300,000 years).

Conclusions

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 3He in the sediment is derived from the IDP component.

To first order, over the past 200 ka, 3He concentration variations correlate well with 230Thxso activity variations which suggests that the flux of extraterrestrial 3He has remained relatively constant. By assuming a constant flux of 230Th, we calculate an extraterrestrial 3He flux of about 9.6x10-16 cm3STP.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 230Thxso/3He ratio indicates changes in either the flux of 230Th, 3He, or both.

Unlike the Th method which is limited to the past 300 ka, normalizing to a constant flux of 3He 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. 3He concentration versus 3He/4He 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. 3He concentration versus 3He/4He ratio. The data approximate mixing between two components, an IDP component (3He~3x10-5 cm3STP.g-1, 3He/4He=2.4x 10-4; [15] ) and a terrigenous component (3He~3x10-15 cm3STP.g-1, 3He/4He=1x 10-8; [3] )

 

 

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

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

 

 

Figure 4. 230Thxso activity versus 3He concentration for equatorial Pacific sediment samples. The best fit line excludes the two bracketed points (explained in text). The correlation coefficient, R2, is 0.70. The positive intercept, 3.5 dpm.g-1, is within 2[[sigma]] uncertainty of zero.