Benthic Mixing in Deep Sea Cores as Determined by 14C Dating and its Implications Regarding Climate Stratigraphy and the Fate of Fossil Fuel CO2 1

Wally's Quest to Understand the Ocean's CaCO3 Cycle

“…The answer is plenty! Another one of my graduate students, George Kipphut, was the first to make the connection (Peng et al 1977). We had been thinking that the radiocarbon age of the CaCO 3 in the bioturbated zone could be approximated by mass, M, of CaCO 3 in the mixed layer divided by the rain rate, R, of CaCO 3 to the sea floor as follows: Because of radiocarbon decay within the mixed layer, the relationship is actually a bit more complicated:…”

“…where τ is the mean lifetime of radiocarbon atoms, i.e., 8.25 kyrs (see Peng et al 1977 for derivation). Take, for example, a situation where the mass of the CaCO 3 in the mixed layer is 6 g cm…”

“…Kipphut pointed out that if CaCO 3 dissolution occured within the bioturbated zone, provided the mixing depth remained unchanged, the radiocarbon age would be reduced (Peng et al 1977). Suppose that dissolution had reduced the mass of CaCO 3 in the mixed layer from 6 g cm −2 to 4 g cm −2 , then the radiocarbon age would be only 1.79 kyr.…”

The modern and glacial thermoclines along the Bahama Banks

“…Based upon the mean ocean o 13 C to phosphate ratio of Broecker and Peng [1982] ( -0.93 per mil/ ~tmol kg-1 ) which has been corrected for this effect, the phosphate concentrations of glacial thermocline waters were from about 0.1 to 0.65 ~tmol kg-1 less than today-the biggest difference occurring at the depth of the modern o 13 C minimum. It is currently estimated that the mean o 13 C of glacial ocean water was rv0.3 per mil less than during the recent Holocene [Curry et al, 1988;Duplessy et al, 1988] due to the transfer of 12 C encriched terrestrial carbon to the oceans [Shackleton, 1977]. Taking this into account, glacial phosphate concentrations within the thermocline must have been 51 from about 0.4 to 0.9 J.LmOl kg-I less than today.…”

An occurrence of Tuhua Tephra in deep‐sea sediments from offshore eastern North Island, New Zealand

“…NZA-numbers correspond to ages in Table 3. in marine sediments may displace "instantaneous" markers downwards by up to the depth of the mixed layer, provided the benthos is not smothered by the event (Manighetti et al 1995). With typical mixing depths of 6-10 cm (Nozaki et al 1977;Peng et al 1977), this process could displace an ash peak by several hundred, or even thousands of years where sedimentation rates are low. In our core, however, the sedimentation rate across the interval containing the Tuhua Tephra is c. 39 cm/cal kyr, and would require a mixing depth of over 20 cm to account for all of the difference between the modelled age in this core and the 7000 cal yr BP (6130-± 30 14 C yr) age reported by Froggatt & Lowe (1990).…”