Holocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods

Earth and Planetary Science Letters 242 (2006) 170 – 185 www.elsevier.com/locate/epsl

Holocene sediment deposition on a NE Atlantic transect including Feni Drift quantified by radiocarbon and 230Thexcess methods J. Thomson a, D.R.H. Green a,*, P. van Calsteren b, T.O. Richter c, T.C.E. van Weering c a

National Oceanography Centre, Southampton, SO14 3ZH, UK Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK Royal Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands b

c

Received 22 April 2005; received in revised form 14 September 2005; accepted 23 November 2005 Available online 3 January 2006 Editor: E. Boyle

Abstract Radiocarbon and 230Thexcess measurements were undertaken on the sediments from fifteen box cores recovered between 48–588N and 12–228W at water depths of 1100–4500 m in the northeast Atlantic. Eight of the cores were from Feni Drift in the Rockall Trough at depths of 1700–2500 m and the remaining seven formed an approximate north/south transect at various water depths from the abyssal plain north on to Rockall Plateau. Mean Holocene sediment accumulation fluxes for all cores were established from profiles of bulk sediment radiocarbon age against depth to be in the range 3–7 cm ky
1 off the Drift and 4–23 cm ky
1 on the Drift. When compared with these radiocarbon-based sediment accumulation rates, precision measurements of 230Th reveal that the 230Thexcess levels present in the sediments match or slightly exceed the potential supply from the vertically overlying water column in a majority of the cores. One core from the open abyssal plain that had the lowest radiocarbon-based accumulation rate in the entire set also had a close balance between predicted and measured 230Thexcess values. With the assumption that only 230 Thexcess produced in the overlying water column is supplied to the sediments, the 230Thexcess values in this core and the other six containing carbonate ooze sediments all imply a rather constant regionally averaged sedimentation flux of 2.0 F 0.2 (1r, n = 7, min. 1.8, max. 2.3) g cm
2 ky
1 rather than matching the 230Thexcess fluxes implied by the radiocarbon data. This singular value is similar to the mean Holocene flux reported for the northeast Atlantic by previous work that utilized the 230Thexcess method, although it is somewhat lower than estimates for the Holocene based on oxygen isotope stratigraphy or radiocarbon methods. The current transport processes that result in the high sediment accumulation rates on Feni Drift have also resulted in a compositional fractionation of the sediments. All sediments from Feni Drift have lower CaCO3 contents than the remainder of the sediments studied, and these are accompanied by lower 230Thexcess specific activities than are found at comparable water column depths elsewhere on the transect. Consistently higher and more variable regional sediment accumulation fluxes are therefore calculated from the Feni Drift 230Thexcess data with the constant flux assumption (average 2.8 F 0.4 g cm
2 ky
1; n = 8, min. 2.3, max. 3.3), some 40% higher than the constant value measured in the other cores. It seems likely that the mean sediment accumulation flux at Feni Drift inside Rockall Trough is consistently higher than on the open ocean margin of the basin, so that the regionally averaged sedimentation fluxes indicated for the Drift by the measured 230Thexcess data are also consistently higher than the singular value measured elsewhere in the northeast Atlantic. D 2005 Elsevier B.V. All rights reserved. Keywords:

230

Th dating; radiocarbon dating; deep-sea sediments; Feni Drift; NE Atlantic Ocean

* Corresponding author. E-mail address: [email protected] (D.R.H. Green). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.034

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

1. Introduction Uranium has a long residence time in seawater, so that there is a constant U concentration in seawater and hence a constant production of its daughter radionuclide 230Th [1–3]. Unlike the U isotopes, however, 230Th is a particlereactive species that is rapidly removed from seawater along with sedimenting material. This results in a short residence time for 230Th in the ocean water column (10–40 y; [4,5]) and as a result the flux of 230Th to the sea floor depends mainly on the depth of the overlying water depth column [6]. A substantial proportion of the 230 Th present in seawater is always found to be associated with the solution (b 0.45 Am) phase, rather than on particulate material (e.g., 85–90% on N0.45 Am material; [7]), however. The undoubted particle reactivity of 230 Th therefore cannot mean that it is irreversibly bound to particles, and 230Th behaviour in the ocean water column is modelled as a reversible equilibrium process (e.g., [8,9]). The fact that 230Th approximates a constant flux tracer for sedimenting material is widely applied in quantification of oceanographic processes, as discussed in recent reviews [10,11]. One application for 230Th data is in the assessment of the efficiency of sediment traps in interception of material settling through the water column, where the amount of 230Th captured by the trap is compared with the amount of 230Th that was produced in the water column overlying the trap during its period of deployment (e.g., [12–14]). Another important application is in the detailed assessment of changes in sediment accumulation over time in sediment cores from the open ocean, because the constant flux criterion means that each 230Th measurement made implies a sediment deposition flux [11,15,16]. Fluxes of other sedimentary components and particle reactive elements in deep-ocean sediments are also often quantified by normalization to 230Th fluxes using the constant flux assumption (e.g., [17–21]). The 230Th method has a further application in a situation where a close balance between production of 230 Th in the overlying water column and its inventory in the underlying sediments certainly does not hold. This is on sediments from drifts or contourites, the constructional deep-sea sedimentary deposits that develop from sustained current action, with the preferential deposition or focusing of sediment into a localized area after erosion upstream and transport downstream. Suman and Bacon [22] proposed that a relative measure of the additional amount of sediment focused into the area and deposited on a contourite might be quantified by a focussing factor, calculated as the amount of 230Th actually present in a sediment section divided by the

171

amount that could have been produced in the overlying water column over the same period of time. Each 230Th datum from sediment drift sediments also returns a flux estimate that Suman and Bacon [22] identified as the regionally averaged sedimentation flux. Francois et al. [16] further considered the sense in which this regional flux might be affected if redistributed sediment were subject to various modifications during transport and redeposition. Unless reequilibration of sediment with the water column 230Th during transport is very efficient, the focusing factor concept cannot be valid if the redistributed sediments are old or if they are supplied downslope from shallower water. At the limit, the sediment focusing application implies that the concentration of 230Th developed in the sediments of a basin must be inversely proportional to the total sediment input to the basin, and that all sediments equilibrate with the amount of 230Th appropriate to the water depth at which they are deposited. These are severe requirements, and Hall and McCave [23] and Lyle et al. [24] have criticised the 230Th sediment focusing application. Suman and Bacon [22] noted that their regionally averaged sedimentation flux might be affected by size sorting of the sediment during current redistribution, and recent work suggests that 230Th associates preferentially with the lithogenic component of sediment rather than the biogenic carbonate or opal components [25], although this remains controversial [26,27]. This work tests the validity of the sediment focusing concept by examination of the rates of 230Th deposition in a set of cores from the northeast Atlantic that includes a sub-set from the Feni Drift contourite (Fig. 1). Mean sediment accumulation rates applicable to the Holocene are first determined from profiles of radiocarbon analyses in the cores. A single high precision 230 Th measurement is then made for each core and the implied sediment accumulation rates compared with the radiocarbon estimates. The advantages of performing this comparison in sediments that are only a few thousand years old are that the radiocarbon data provide estimates of sediment accumulation that are based on assumptions fully independent of those applied to 230Th. The radioactive ingrowth and decay uncertainties that apply in older sediments are also minimized because of the long half-life of 230Th (75.69 ky, [28]) and the short time since the sediments were laid down. 2. Methods This work is based on a set of 15 box cores (Fig. 1), 7 of which are from water depths of 1700–2500 m on Feni Drift on the western side of Rockall Basin collected

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J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

Fig. 1. Map of the core locations in the northeast Atlantic Ocean.

during the EU European North Atlantic Margin (ENAM) project. The remaining cores are from depths of 1100– 4500 m, 2 from Rockall Bank and Feni Drift taken during the UK Benthic Boundary Layer Experiment (BENBO; [29,30]), and 6 from the Porcupine Abyssal Plain and the terrain south of Rockall Bank taken during the UK Biogeochemical Ocean Flux Study (BOFS; [31]). Multiple radiocarbon data were available for all cores, all of which were based on CO2 liberated by acid treatment of whole dried ground sediment. Data for the BOFS and BENBO cores were reported by Thomson et al. [29,31,32], and were gathered by radiometric measurement of benzene prepared from large (10–20 g) sediment samples. Analyses on the ENAM cores were made on graphite targets prepared from small (30–40 mg) samples of sediment at the NERC Radiocarbon Laboratory and measured by accelerator mass spectrometry at the University of Arizona NSFAMS facility (AA-55248-AA-55302). For reasons that will be explained below, a single sample from ~10 cm depth in each core was selected for high precision 230Thexcess measurement by thermal ionization mass spectrometric (TIMS) analysis. The composition of the bulk sediment of these samples was also determined by fusing 100 mg of the finely-ground sediment with 500 mg LiBO2 flux at 1000 8C to ensure total sample dissolution [33]. The fused flux was

dissolved in HNO3 and the solution was measured for major and minor elements by inductively coupled plasma-atomic emission spectrometry (ICP-AES) with a Perkin Elmer Optima 4300 V instrument by reference to matrix-matched standard solutions. For TIMS analysis, 500–700 mg sediment was dissolved with HNO3, HF and HClO4 acids and a mixed 229Th / 236U spike was added to the totally dissolved samples. Uranium and thorium fractions were separated on 2 ml anion exchange columns using standard techniques [34], loaded on to graphite-coated Re filaments, and analysed on a Finnigan MAT262 mass spectrometer equipped with a retarding potential quadrupole and secondary electron multiplier [35]. A dynamic peak switching routine was employed to measure 234 U / 236U and 235U / 236U (with 235U as a proxy for 238 U assuming an invariant natural 238U / 235U mass ratio of 137.88), and 230Th / 229Th and 232Th / 229Th. The data with associated 2 j uncertainties are presented in Table 2, where ratios in parenthesis are activity ratios. Uncertainties were propagated from the withinrun uncertainties, weighing uncertainties and uncertainties in spike concentrations and isotopic compositions, but not uncertainties in the 230Th and 234U half-lives where the values of Cheng et al. [28] were used. Two procedure blanks run in parallel with samples yielded 105 and 60 pg for 238U and 232Th, respectively,

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

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Fig. 2. Radiocarbon age versus depth profiles for the seven ENAM cores. The mean surface mixed layer ages (T SML), sediment accumulation rates and mixed layer depths indicated were calculated from the lines fitted to these profiles as described in the text.

typical values for sediment analysis with HF and HClO4 dissolution. These values represent between 0.1–2% and 0.02–0.13% of the sample measurements for 238U and 230Th, respectively. Two in-house standards were used to assess external reproducibility: (i) A natural uranium standard spiked with 236U (bU456 std3Q) with a long-term reproducibility of 0.2% (2r, n = 100) and 0.5% (2r, n = 100) for 235U / 236U and 234U / 236U measurements, respectively. The (234U / 238U) activity ratio had an uncertainty of 0.45% (2r, n = 100) when calculated from the measured ratios. (ii) A thorium standard (bCP230/229Q) that is a mixture of 230Th and 229Th spikes designed to reproduce the 230Th / 229Th ratio of a young carbonate sample. A reproducibility of 0.5% (n = 73) was achieved for this standard. 3. Results and discussion 3.1. The study area Rockall Trough is a SW–NE trending basin that lies between the continental margin of the British Isles to the

east and Rockall Bank to the west (Fig. 1). The Trough shoals from the Porcupine Abyssal Plain at 4500 m in the southwest towards the northeast where it is enclosed by the Wyville–Thomson Ridge and various seamounts at ~500 m depth with intervening sills at ~1200 m. The flow of water over and in the Trough is complex and variable at different depths [36]. Physical analysis of water flow inside Rockall Trough, through both observations [37–39] and modelling [40] suggests that the mean organized flows are weak and stratified but that the circulation must operate in different directions at different depth levels in the basin, with a cyclonic flow below ~1200 m. Analysis of the morphology of sedimentary features around Rockall Trough indicates that sediment is supplied into the basin from the upper slopes on both margins, with large mass failures from Rockall Bank to the west but with the sediment flux channelled through canyon, channel and fan systems in the east, particularly in glacial times [41]. In contrast to the indications of modest currents from modern water column measurements within the basin, the sedimentary evidence shows clear signs that the morphology in the basin has been shaped by near bottom flows that are powerful enough to move sediment. A deep cyclonic

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J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

circulation is inferred with currents flowing northwards on the eastern side of the Trough and southwards on the western side to redeposit sediments from the eastern on to the western margin [42,43]. Feni Drift is the contourite that drapes the western side of Rockall Basin under the influence of this deep circulation [44]. Despite evidence of occasional downslope failures in glacial times and sustained parallel-to-slope depositional processes, many paleoceanographic studies of climate variability have been undertaken on cores from Feni Drift because records of good temporal resolution can be obtained from the high sediment accumulation rates encountered (e.g., [45–49]). Such studies are important because the Drift is located just north of the polar front in glacial times. 3.2. Establishment of mean Holocene sediment accumulation rates from radiocarbon data Radiocarbon data are used to model the sediment accumulation rates and the differences in the radiocarbon age of sediment deposited at the different core sites. Profiles of bulk carbonate radiocarbon ages versus depth are often readily interpreted in northeast Atlantic sediments (e.g., [31,50–52]) because they appear to be free from the complications caused by carbonate dissolution effects that can occur elsewhere (e.g., [53,54]). Northeast Atlantic radiocarbon profiles typically exhibit a surface mixed layer (SML) that has a near-constant age from the surface down to a depth of 5–20 cm as a result of mixing by bioturbation. A zone where radio-

carbon age increases as a linear function of depth underlies the active bioturbation of this SML. The steady-state box model of Erlenkeuser [55] enables estimates of (i) the mean age and depth of the SML, (ii) the mean sediment accumulation rate in the underlying zone and (iii) the radiocarbon age of newlydeposited sediment at the sea floor (T 0). In order to apply the Erlenkeuser model, a subjective judgment is first required to estimate the depth level that separates the active SML above from the unbioturbated region below. A linear best-fit regression of radiocarbon age on depth is then made on the sub-set of data below this level to determine the mean sediment accumulation rate. Calculation of the depth at which the mean mixed layer age intercepts this regression line then refines the initial estimate of mixed layer depth. Treatment of the new data for the ENAM cores in this manner is illustrated in Fig. 2 and the estimated accumulation rates, mixed layer depths and T 0 values are presented in Table 1 along with the corresponding published estimates from the other cores [29,31,32]. The extremes in mean sediment accumulation rate are low values (3–4 cm ky
1) in two cores (11880 and 11881) from the Porcupine Abyssal Plain and high values (15–23 cm ky
1) in three Feni Drift cores (ENAM 96-03, 96-05 and 96-06) from water depths between 2100–2550 m. The remaining ten cores, including the six others from Feni Drift, exhibit rates of 4–7 cm ky
1. Similarly variable sediment accumulation rates on a local scale on Feni Drift have also been noted in previous investigations. As examples, Keigwin and Jones [44] reported a core (KNR51 GGC11; 548

Table 1 Locations and statistics of the cores studied Core

BENBO Bi ENAM96-01 ENAM96-02 BENBO Cii ENAM96-03 ENAM96-04 ENAM97-09 11884#2BX ENAM96-05 ENAM96-06 11889#4BX 11882#3BX 11886#4BX 11881#3BX 11880#5BX a b

Latitude

Longitude

Water depth

14

(8N)

(8W)

(m)

(cm ky
1)

(cm)

(yr)

(yr)

578 25.6V 55.69328 55.68178 578 06.0V 55.60088 55.5388 54.89328 518 44.4V 55.46288 55.64978 538 42.2V 508 40.7V 528 33.0V 498 47.5V 478 46.5V

158 41.1V 14.80188 14.77838 128 31.0V 14.61578 14.48878 16.58888 228 35.0V 14.33238 13.98788 218 19.6V 218 51.5V 228 06.0V 218 14.0V 198 42.8V

1098 1692 1754 1925 2091 2258 2452 2365 2483 2543 3275 3547 4005 4067 4540

4.4 6.2 4.8 6.5 16 3.9 5.9 4.3 19 23 5.9 6.6 5.9 3 3.5

14 11 11 17 18 18 11 0 7 13 9 9 9 9 9

2600 2800 2900 3000 2700 3010 3000


510 1200 960 910 1630 1770 1300 1200 1990 1990 1000 780 950 410 560

C sed. rate

SML= Surface mixed layer. T 0 = modelled radiocarbon age of newly-deposited sediment at the sea floor [55].

SMLa depth

SMLa mean age

2300 2500 2390 2030 2350 2310 2660

T 0bage

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

Fig. 3. (a): Mean mixed layer age calculated for all cores as a function of derived sediment accumulation rate. Data for cores from Feni Drift are circled. The dotted curves are the expected T SML values for the indicated SML depths according to the relationship of Berger and Heath [57] defined in the text. (b): T 0 (modelled radiocarbon age of newly-deposited sediment at the sea floor) values calculated for all cores according to the relationship of Erlenkeuser [55] as a function of derived sediment accumulation rate. Data for cores from Feni Drift are circled. The dotted curves are model calculations that assume that (i) the first 3 cm ky
1 of sediment accumulation has T 0 equal to the surface ocean radiocarbon age of 400 yr but that (ii) all sediment added at N3 cm ky
1 has T 0 ages of 1000, 1500, 2000, 2500 or 3000 yr as indicated on the curves. The range of T SML values actually found in the cores (3a, lower panel) is shaded.

28.4VN, 158 14.8VW; 2680 m) with a Holocene accumulation rate of 5 cm ky
1, whereas Duplessey et al. [45] found a rate of ~12 cm ky
1 in core CH73-139C (548 38TN, 168 21TW; 2209 m), and a rate of ~25 cm ky
1 may be calculated from the 14C data of Oppo et al. [56] for ODP Site 980 (558N, 158W; 2179 m; core 1H01). The estimated SML depths for fourteen of the fifteen cores reported here range from 7–18 cm. At steady state with a well-mixed surface layer, the depth and age of the SML are related by [57]:   h ¼ ss et=s
1

ð1Þ

where h and t are the depth and mean age of the SML, s is the mean life of radiocarbon and s is the sediment accumulation rate. Although the measured SML age is in the narrow range of 2000–3010 yr in all cores, such

175

SML ages are impossibly old for the three most rapidly accumulated sediments in particular according to the Berger and Heath [57] relationship (Fig. 3a). Carbonate sourced directly from the surface ocean should have an initial age T 0 = 400 yr [58], and calculation of T 0 for each core by the Erlenkeuser [55] box model reveals that the fresh material arriving at the sites with lower accumulation rates is younger (400–1300 yr for 9 of the cores, Fig. 3b) than the older (1600–2000 yr) material that arrives at the three sites with the highest accumulation rates. The radiocarbon data used here were derived from whole sediment (rather than from planktonic foraminifera separated from the large size fraction), and therefore represent the mean age of all CaCO3 in the sediment regardless of origin or size. While a T 0 value of 2000 yr could be produced by a mixture of 18% of infinitely old (N 50,000 yr) CaCO3 with 82% of modern CaCO3 with a surface ocean age of 400 yr, it seems more likely that the observed T 0 ages of around 2000 yr at the sites with the highest accumulation rates are the result of an admixture of a larger fraction of SML material from elsewhere in the basin with its mean age of 2000– 3000 yr (Fig. 3a). Thomson et al. [29] demonstrated that the higher accumulation flux observed on Feni Drift (BENBO site C) exceeded that on the crest of Rockall Plateau (BENBO site B) because of an increased flux of material in size classes b 20 Am, while fluxes of N 20 Am material were similar. Material b 20 Am in size will include redistributed coccolith and comminuted carbonate that will contribute to the measured radiocarbon age. A fit to most of the box core T 0 data can be achieved if it is assumed that the first 3 cm ky
1 of sediment accumulation has the surface ocean radiocarbon age of 400 yr (as measured in core 11881), but that all additional material that is added at N3 cm ky
1 has an age of 1500–2500 yr (Fig. 3b). Coccolith-derived alkenones from the fine fraction of sediments of other drifts have been shown to have radiocarbon ages that are consistently older by a few thousand years than co-existing foraminiferal carbonate of larger size [59,60]. The conclusion here that the enhanced flux of material deposited on Feni Drift is finer and slightly older than new surface ocean production is similarly consistent with radiocarbon data from a piston core retrieved at the same site as one of three box cores with a very high accumulation rate (ENAM 96-06). Over the 0.5 m depth range where the ENAM 96-06 box and piston cores overlap, the AMS radiocarbon ages of samples of the planktonic foraminifer G.bulloides separated from the piston core are consistently ~1500 yr younger than the bulk CaCO3 AMS radiocarbon ages measured in the box core (Fig. 4). Note that

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J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

Fig. 4. Comparison of bulk radiocarbon ages determined for ENAM 96-06 box core with radiocarbon ages of G.bulloides foraminifera separated from piston core ENAM 96-06 at the same position. Note that the foraminiferal ages determined near the top of the piston core are younger than the bulk ages in any of the box core SMLs (Fig. 3), but that forminiferal CaCO3 forms a small proportion of the total sediment. Two off-trend analyses from thin (cm scale) bioturbated turbidites in the piston core are also shown.

foraminiferal CaCO3 is in the large size range and makes only a small contribution to the total sediment mass, and that the foraminiferal radiocarbon ages measured near the top of this piston core are younger than the bulk CaCO3 radiocarbon ages measured in the SMLs of any of the box cores studied (Fig. 3 and Table 1). 3.3. Compositional variations in the sediments as a function of location While the radiocarbon data indicate that redistributed SML material may provide the extra sediment that is accumulated on Feni Drift, size sorting of material during current transport has also resulted in a compositional fractionation. By comparison with sediments elsewhere in the basin, the sediments of Feni Drift are muddier and finer-grained [29,44,61]. The lower CaCO3 content of all Feni Drift sediments is apparent when total Ca ICPAES analyses are converted to the equivalent amount of

CaCO3 (Fig. 5). Stoll and Schrag [62] have demonstrated that the Sr / Ca weight ratios in cleaned biogenic calcite from foraminifera and coccoliths are different at ~0.003 and ~0.005, but there is no clue to any related change in the composition of CaCO3 in these cores because the whole sediment Sr / Ca analyses have a consistent mean Sr / Ca value of 0.0044 F0.0002 (1r). When normalized against Al to take account of the dilution produced by CaCO3 content variations [63], certain elements such as Fe, Ti and Zr show a common variation that may be related to the water depths at which the cores were taken (Fig. 5). Increases in Alnormalized Zr, Ti and Fe suggest that increased amounts of heavy minerals such as zircon (ZrSiO4), rutile (TiO2) and ilmenite (FeTiO3) may be relatively more abundant in the sediments from shallower water depths. Alternatively, however, it may be that these data are more influenced by sediment provenance so that a display against water depth is somewhat misleading. Lonsdale and Hollister [42] and Richter et al. [64] have inferred that there must be a contribution of material from the Irish margin to the sediments on Feni Drift. The variation in Zr / Al, Ti / Al and Fe / Al ratios may reflect some fractionation of material arriving from the margins into the deep basin, i.e., with the contribution of heavy minerals greater at shallower depths closer to source which may imply a source on Rockall Plateau instead. Whatever the cause, the importance of these element / Al variations for this study is that heavy minerals tend to have higher concentrations of U and Th incorporated in their structures when compared with lighter minerals such as quartz or feldspar (e.g., [65]). 3.4. 230Thexcess specific activities in the sediments and fluxes implied by the constant flux assumption The total 230Th measured in sediments comprises detrital 230Th (assumed to be in radioactive secular equilibrium with its parent 234U), 230Thexcess produced in the oceanic water column and added to the sediments during deposition, and possibly 230Th ingrown from authigenic U taken up by the sediments after deposition. If the authigenic U content is negligible, the specific activity of 230Thexcess in the sediments is defined as: 230

  Thexcess ¼ measured specific activity of 230 Th   234
measured specific activity of U :

ð2Þ

The 230Thexcess calculated in this manner is then corrected to the value at the time of deposition by: 230

Thexcess


0

¼

230


Thexcess =e
230t :

ð3Þ

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

177

Fig. 5. Sediment accumulation rate (cm ky
1), all Ca expressed as CaCO3 (wt.%), Sr / Ca mass ratio and Al normalized mass ratios for K, Si, Fe, Ba, Y, Zr and Ti for all cores displayed as a function of core water depth. The depths of the Feni Drift cores are shaded.

where k 230 is the decay constant of 230Th and t is the time elapsed since deposition of the sediment. In the present case, conversion of the 230Thexcess data to (230Thexcess)0 values corresponding to the instant of deposition is complicated because of the effects of bioturbation in the SML. If the radiocarbon age of the mixed layer (T SML) of 2000–3000 yr is accepted, the measured 230Thexcess data would be expected to have decayed to 0.98–0.97 of their (230Thexcess)0 values. Calculated 230Thexcess data without amendment are taken as reasonable approximations of (230Thexcess)0 values in this case. If authigenic U is present in the sediments, an alternative approach based on the assumption of a constant (238U / 232Th) activity ratio in the detrital phase is necessary to calculate 230Thexcess [66]: 230

Thexcess ¼½measured specific activity of

230


f ½measured specific activity of

Th 232

Th


1:14ð½measured specific activity of
f ½measured specific activity of

232

238

U

ThÞð1
e
230T Þ ð4Þ

where f is the lithogenic (238U / 232Th) activity ratio and T is the time elapsed since uptake of authigenic U. T will be slightly less than t from Eq. (3) because authigenic U is added after sediment deposition. In expression (4), the second term on the right represents the detrital contribution of 238U that is assumed to be in radioactive secular equilibrium with 230Th, and the third term represents the ingrowth of 230Th with authigenic 234U that was initially supplied with the seawater (234U / 238U) activity ratio of 1.14. The lithogenic (238U / 232Th) activity ratio is often ~1 which corresponds to a Th / U mass ratio of 3.1. The theoretical input of 230Thexcess to the sediments from the vertically overlying water column only is: 230


Thexcess flux d:p:m: cm
2 ky
1 ¼ bz ¼ 0:00267z

ð5Þ

where d.p.m. is disintegrations min
1 g
1, h is the production of 230Th in d.p.m. cm
2 ky
1 from the 2.835 d.p.m. kg
1 234U present in seawater at a salinity of 35 [3], and z is the water column depth in metres. If all this production of 230Thexcess in the vertically overlying water column were supplied quantitatively to the

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sediments (the constant flux assumption), then the specific activity of 230Thexcess expected in the sediments can be predicted from the radiocarbon sediment accumulation rates and dry sediment bulk density: predicted

230

Thexcess d:p:m: g
1





¼230 ThðexcessÞ flux=sediment flux gcm
2 ky
1 ¼ bz=sq

ð6Þ

where s is the sediment accumulation rate determined from the radiocarbon profiles in cm ky
1 , and U is the dry bulk sediment density in g cm
3 (assumed constant at 0.65 g cm
3 for all cores, Fig. 6). 3.5. Implications of 230Thexcess specific activities for sediment accumulation rates in cores located off Feni Drift A sample from ~10 cm depth was selected to represent each box core for precision 230Th measurement for several reasons: (i) the highest water contents and hence lowest dry bulk densities occur in surficial sediments from

Fig. 6. Dry bulk density profiles versus depth for cores 11880, BENBO C and BENBO B. These cores are the most southern (11880) and most northern (BENBO B) on the transect, include a Feni Drift example (BENBO C), and cover the full range of CaCO3 contents encountered (Fig. 5). Density profiles were not available for the ENAM cores, and this plot is taken to justify the assumption of a mean density of 0.65 g cm
3 for the sample at ~10 cm from all cores.

b 10 cm (Fig. 6), and one aim was to obtain samples with similar seawater salt contents and dry bulk densities (ii) given the requirement for sediments that were as young as possible to minimize radioactive ingrowth and decay assumption uncertainties, the active bioturbation revealed by the 14C profiles means that samples from the SML are the youngest that can be obtained (iii) 10 cm is generally in or towards the base of the sediment SML (Table 1), so that the sediment pore waters at this depth are expected either to be oxic, or at least to have turned anoxic only in the very recent past. Since post-depositional uptake of authigenic U from seawater occurs in the sediments of this area in post-oxic conditions [30], selection of samples from 10 cm aimed to avoid or minimize authigenic U content (iv) previous lower precision radiometric measurements on some of these cores [29,30,31] have shown that 230Th levels alter little with depth (see also Fig. 10 that is discussed later), and single (230Thexcess)0 measurements are regularly used to determine sediment fluxes [11]. There is an obvious distinction between the Th / U mass ratio of ~5 measured in the CaCO3-rich, Zr-poor sediments of the cores from the plain (11880, 11881, 11882 and 11886; Figs. 5 and 7) and the lower Th / U values of 2–3 measured in the relatively CaCO3-poor, Zr-rich sediments from Feni Drift. This difference necessitates somewhat different procedures to convert the measured 230Th values to 230Thexcess values for the CaCO3–rich sediments and for the muddier Feni Drift sediments. Unlike the Feni Drift sediments that will be discussed below, there appears to be little or no authigenic U in the high CaCO3 sediments because the Th / U ratio does not change appreciably with depth in these cores [31] and the (234U / 238U) activity ratios are ~1.00 (Table 2). Measured 234U levels and Eq. (2) were therefore used to convert total measured 230 Th data to 230Thexcess specific activities for these cores, except for core BENBO B that has a high Zr content (Fig. 5) and which was treated like the Feni Drift samples as described below. These corrections to the total 230Th specific activities are of a lesser magnitude in the CaCO3-rich sub-set of the cores because they have relatively low U contents, and the deepest cores also have relatively large 230Thexcess specific activities from the longer overlying water column. When total 230Th, calculated 230Thexcess specific activities, and the 230Thexcess values predicted from

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

179

Fig. 7. Th / U mass ratio (measured by TIMS) plotted versus Zr (measured by ICP-AES and calculated on a carbonate-free basis) for all cores.

radiocarbon accumulation rates through Eq. (6) are compared (Fig. 8), the predicted 230Thexcess specific activities from the radiocarbon accumulation rates are consistently less than the 230Thexcess values calculated from the measurements. All the sediments with high CaCO3 contents from the north–south transect therefore contain more 230Thexcess than could have been supplied

from the overlying water column because of the inverse nature of Eq. (6), with the exception of the data from core 11881. As well as displaying a close balance between predicted and measured 230Thexcess values, this was the core with the lowest radiocarbon sediment accumulation rate in this set of cores and with its T 0 age closest to the surface ocean value of 400 yr (Table 1).

Table 2 Thermal ionisation mass spectrometric analyses of the representative samples from each core Sample name

z (m)

238 U (Ag g
1)

F 2r

234

U (pg g
1)

F 2r

(234U / 238U)

F2r

230

Th (pg g
1)

F2r

232

Th (Ag g
1)

F 2r

C07 BENBO Bi 9.5–10 cm ENAM 96-01 BX 10 cm ENAM 96-02 BX 10 cm CD107 BENBO Cii 9.5–10 cm ENAM 96-03 BX 10 cm ENAM 96-04 BX 10 cm 11884#2BX 9.0–9.5 cm ENAM 97-09 BX 10 cm ENAM 96-05 BX 10 cm ENAM 96-06 BX 10 cm 11889#4BX 10–11 cm 11882#3BX 9.0–9.5 cm 11886#4BX 10–11 cm 11881#3BX 9.0–9.5 cm 11880#5BX 10–11 cm

1100 1692 1754 1925 2091 2258 2365 2452 2483 2543 3275 3547 4005 4067 4540

0.4659 1.0858 0.9991 1.2702 1.9634 1.8109 0.2566 0.7781 1.9195 2.5845 0.5165 0.3138 0.4840 0.3077 0.3549

0.0009 0.0026 0.0022 0.0028 0.0035 0.0034 0.0005 0.0016 0.0041 0.0051 0.0010 0.0005 0.0010 0.0006 0.0007

0.02689 0.06178 0.05599 0.06805 0.11106 0.10180 0.01475 0.04597 0.10761 0.14778 0.02719 0.01718 0.02557 0.01663 0.01899

0.00020 0.00039 0.00047 0.00048 0.00063 0.00059 0.00009 0.00034 0.00062 0.00084 0.00017 0.00010 0.00017 0.00011 0.00015

1.069 1.054 1.039 0.993 1.048 1.042 1.065 1.095 1.039 1.060 0.975 1.015 0.979 1.001 0.992

0.008 0.007 0.009 0.007 0.006 0.006 0.007 0.008 0.006 0.006 0.006 0.006 0.007 0.007 0.008

0.0395 0.0397 0.0403 0.0634 0.0673 0.0619 0.0655 0.0513 0.0692 0.0733 0.1030 0.1103 0.1397 0.1209 0.1466

0.0005 0.0007 0.0008 0.0007 0.0008 0.0011 0.0008 0.0006 0.0014 0.0011 0.0012 0.0017 0.0015 0.0015 0.0015

1.13 2.01 2.22 4.05 3.70 3.86 0.84 2.10 4.20 4.33 1.74 1.55 2.33 1.58 1.86

0.10 0.18 0.20 0.36 0.33 0.34 0.07 0.18 0.37 0.39 0.19 0.14 0.21 0.14 0.16

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J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

derived by Cremer et al. [67,68] from radiocarbon and isotopic stratigraphy and is comparable with the rates derived from the radiocarbon data in this work. The 230Thexcess-based flux estimates are however similar to previous Holocene rates that have been reported in all known (230Thexcess)0 investigations for the deep NE Atlantic. As examples, rates ~2 g cm
2 ky
1 for the Holocene were reported by Thomson et al. [69] in core CD63#9 (468 23.8VN, 128 32.8VW; 3849 m water depth) and by McManus et al. [70] in core V28-82 (498 27VN, 228 16VW; 3935 m water depth). The value of ~2 g cm
2 ky
1 as recorded in the sub-set of cores located off Feni Drift therefore appears to be a good representation of the regionally averaged sedimentation flux [22] in the NE Atlantic implied by 230 Thexcess data in the Late Holocene. 3.6. Implications of 230Thexcess specific activities for sediment accumulation rates from Feni Drift cores

Fig. 8. Total measured 230Th specific activities (F2r, triangles), 230 Thexcess specific activities calculated by two different methods as described in the text (filled squares) and the 230Thexcess specific activities predicted with the constant 230Thexcess flux assumption and the radiocarbon accumulation rates of Table 1 at a dry bulk density of 0.65 g cm
3 (open circles) for all samples. Three lines for the constant 230Thexcess flux assumption predicted at fluxes of 2, 3 and 5 g cm
2 ky
1 and the equivalent accumulation rates (cm ky
1) are also shown. The depths of the Feni Drift cores are shaded.

With the eight Feni Drift cores excepted, the Thexcess specific activity calculated in the remaining seven cores from water depths between 1100 and 4500 m appears to be a near linear function of core water depth, regardless of the (true) sediment accumulation rate determined by the 14C method (Fig. 8). As a consequence, the sediment accumulation flux implied by the measured 230Thexcess values and the constant flux assumption (i.e., with sediment flux is treated as the unknown in Eq. (6), also return similar values (Fig. 9). For these cores, the 230Thexcess data correspond to an average sediment flux of 2.0 F 0.2 (1r, n = 7, min. 1.8, max. 2.3) g cm
2 ky
1, which is equivalent to ~3 cm ky
1 at a dry bulk sediment density of 0.65 g cm
3. This constant flux implied by the 230Thexcess data for the sediments of the north–south transect through Eq. (6) is clearly at odds with the varied sediment accumulation rates determined from the 14C data (Fig. 9). It is also considerably lower than the mean Holocene estimate of 4 g cm
2 ky
1 for the northeast Atlantic that was

The presence of additional U in the form of both authigenic and heavy mineral components is suspected in the Feni Drift cores, so that correction of

230

Fig. 9. Sediment fluxes implied by the calculated 230Thexcess specific activities and the constant flux assumption (filled squares) and sediment fluxes corresponding to the radiocarbon sediment accumulation rates of Table 1 at a constant dry bulk density of 0.65 g cm
3 (open circles). The depths of the Feni Drift cores are shaded, and the dotted grids demonstrate that the implied fluxes from the 230Thexcess data fall into two distinct populations of cores collected on and off Feni Drift.

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

181

Fig. 10. Profiles of total 230Th (d.p.m. g
1) and Th / U mass ratio (both alpha spectrometry data) and Zr / Al (ICP-AES data) versus depth for core BENBO Cii from Thomson et al. (2001) [30]. The crosses are the values for the single sample of this core that was analysed by TIMS and ICP-AES in this work.

measured 230Th to 230Thexcess specific activities requires Eq. (4). Measured 230Th specific activities will require a correction for the full amount of 238U present in heavy minerals, but in these sediments authigenic 238U will have been present for only for a short time so that ingrowth of the 230Th daughter in this fraction is expected to be minimal and will be discounted. An evaluation of f for Eq. (4) is usually made from the (238U / 232Th) activity ratio in samples that do not contain authigenic U, if available [66,71,72]. It was noted above that the higher Zr / Al and Ti / Al ratios observed in all Feni Drift cores and BENBO core B (Fig. 5) raised the possibility that these sediments contained an increased heavy mineral content, and indeed low Th / U values are measured in all the sediments with high Zr contents (Fig. 7). It is unlikely that U in heavy minerals is the sole source of the additional U and low Th / U mass ratios in the Feni Drift sediments, however, because (234U / 238U) activity ratios slightly N 1.00 are also observed in the high-Zr sediments (Table 2). Full profiles of alpha spectrometric U and Th data and ICP-AES data are

available for the BENBO site C core [29,30] that reveal that the total 230Th specific activity and Zr / Al profiles are near constant with depth while the Th / U mass ratio falls sharply over the upper 20 cm (Fig. 10). This fall in Th / U is unlikely to result from an increasing zircon content with depth because Zr / Al is invariant in this core. Instead it is likely due to the demonstrated uptake of authigenic U along with other redox-sensitive elements in anoxic conditions at shallow depth [30]. Although this means that sample selection at ~10 cm was not completely successful in avoiding all authigenic U in all cores, this authigenic U is not expected to have had time to develop an appreciable 230Th daughter contribution through radioactive ingrowth. The Th / U mass ratios measured in the uppermost sediments of the BENBO site C core (Fig. 10) were therefore selected to correct all the Feni Drift and the BENBO B sediments (Th / U mass ratio = 4.3, (232Th / 238U) activity ratio = 1.4, (238U / 232Th) activity ratio = 0.73). The magnitude of the corrections to the total 230Th data for the Feni Drift sediments are relatively larger than for the other cores because they have higher Th and U contents

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J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

Fig. 11. Focusing factor for all cores as a function of depth, with the depths of the Feni Drift cores shaded. The focusing factor is defined as [measured 230Thexcess specific activity] divided by [230Thexcess predicted by the constant flux assumption and the rates of sediment accumulation derived from the 14C profiles with a sediment density of 0.65 g cm
3] (Eq. (6)).

because CaCO3 contents are lower (Fig. 4), and lower 230 Thexcess specific activities because of the shallower water depths (Fig. 8). Like the other cores from the transect, the levels of 230 Thexcess estimated in the Feni Drift cores are incompatible with the 230Thexcess supply predicted from the vertically overlying water column alone and the radiocarbon sediment accumulation rates, especially in the three cores with the very high radiocarbon-based accumulation rates (Fig. 8). In these cases, the implied fluxes from the 230Thexcess values are both over- and underestimated by the corresponding values predicted from the radiocarbon-based sediment accumulation rates (Fig. 8), and the sediment fluxes implied by the 230 Thexcess levels in the Feni Drift cores do not correspond to the flux rate of ~2 g cm
2 ky
1 calculated for the other seven transect cores either (Fig. 9). It appears that the current-driven processes that have redistributed the sediments (Fig. 3) and changed their overall composition (Fig. 5) have also altered the consistent relationship of 230Thexcess with depth (Fig. 8) that led to the

singular implied sediment accumulation rate in the other cores (Fig. 9). As a result of the inverse relationship of 230Thexcess values with sediment accumulation rate, the mean implied sediment accumulation rate for the eight Drift cores are consistently higher at 2.8 F 0.4 (n = 8, min. 2.3, max. 3.3) g cm
2 ky
1, and more erratic than the estimates for other seven cores. One possibility is that the lower 230Thexcess specific activities observed in Feni Drift sediments are the result of the short residence time of 230Thexcess in the water column coupled with sustained enhanced local sediment deposition. If this is the case, then the regionally averaged sedimentation flux [22] seems to be consistently ~40% higher on the western margin of Rockall trough compared with the wider NE Atlantic. The different fluxes indicated by the 230Thexcess data may then simply be a result of a larger sediment flux inside Rockall Trough compared with that found outside or on the margins of the Trough and hence further from land/strong currents. In this case it follows that the regionally averaged sedimentation flux cannot have a singular value in this basin. Note that, if all the U measured in the Feni Drift sediments had been assumed to be detrital, so that Eq. (2) rather than Eq. (4) was suitable for the corrections to the total 230Th measurements, then the 230 Thexcess specific activities obtained would all have been lower and the implied sediment accumulation fluxes on the Drift would all have been even higher. It must also be recognised that all the sites sampled on Feni Drift do not experience contourite deposition at the present time, in the sense of receiving a significantly enhanced lateral input of material delivered by bottom currents. Besides small-scale spatial variability of sediment accumulation rates on Feni Drift, seismic records suggest a general decrease in accumulation from north to south [73,74]. The highest sediment accumulation rates across the Drift in the southeast to northwest direction are expected on the ridge crest rather than on the ridge flank, and along the Drift from north-east to south-west the highest rates are expected close to the present-day ddepocentreT at ~568N that was traversed on the ENAM 96 depth transect. From the seismic records, site BENBO C is interpreted as on an erosive part of Feni Drift, sites ENAM 96-01 and 96-02 are on the slope of Rockall Plateau above the crest of Feni Ridge recognised on seismic records, and site ENAM 97-09 is from the lower flank of Feni Drift well south of the depocentre. Within the set of cores from Feni Drift, only three (ENAM 96-03, 96-05 and 96-06) therefore appear to be active contourite sites at present (Figs. 3 and 11).

J. Thomson et al. / Earth and Planetary Science Letters 242 (2006) 170–185

3.7. Summary and implications The underlying assumption in this work is that the radiocarbon accumulation rate provides a reliable estimate of the mean sediment accumulation rate for each core. The systematics of the radiocarbon age of CaCO3 delivered to the sediments (Figs. 3 and 4) then suggests that all the cores contain an additional component of redistributed older sediment. The exception is the abyssal plain core 11881 that has the lowest radiocarbon accumulation rate, the T 0 value closest to the surface ocean reservoir age, and also a close balance between the predicted and measured 230Thexcess. The 230Thexcess specific activity of core 11881 nevertheless delivers an implied sediment flux similar in magnitude to those of the other six cores with high CaCO3 contents from the north–south transect. These seven cores therefore provide corroboration of the proposal of Suman and Bacon [22] that deposited sediment is labelled with 230Thexcess as a predictable function of water depth. As this sub-set of the cores was included in the study to establish a background pattern of sediment accumulation along the transect rather than to seek evidence of sediment focussing, it might have been expected that the measured 230 Thexcess values would reflect true sediment accumulation rates when used with expression (6) above [11,15,16]. Instead it appears that sediment redistribution must be widespread on this transect. If a reliable independent chronology is available from some other method (e.g., oxygen isotope stratigraphy beyond the range of radiocarbon dating), then (230Thexcess)0 data from older sediments of this type may be used to establish past regionally averaged sedimentation fluxes. The sub-set of eight Feni Drift cores was selected specifically to test the application of 230Thexcess to sediment focussing on a contourite. The sediment that is redeposited on Feni Drift has a different composition from the high-CaCO3 content sediments as well as a different mean size [29], and this size and/or composition fractionation also appears to have modified the 230 Thexcess labelling in the Feni Drift sediments (Fig. 8). Luo and Ku [25] estimated K d (g/g) values of 230 Th in the open ocean of 2.3  108 for lithogenic material, 1.0  106 for carbonate and 2.5  105 for opal. From these K d values, relatively high 230Thexcess values in the lower CaCO3 content sediments deposited on Feni Drift might be expected, but the reverse is in fact found (Fig. 8). All the 230Thexcess data from the Feni Drift cores imply fluxes that are ~40% higher than the singular regionally averaged sedimentation flux value found for the high-CaCO3 sediments on the north–south

183

transect, and these 230Thexcess-derived fluxes mostly underestimate the (true) sediment accumulation fluxes derived from the radiocarbon measurements (Fig. 9). As a result, whereas the focusing factors (defined here as [measured 230Thexcess] / [230Thexcess predicted by the constant flux assumption and the radiocarbon sediment accumulation rates at a sediment density of 0.65 g cm
3]) for the high CaCO3 content sediments lie between 0.95 and 2.2 (Fig. 11). The focussing factors for the Drift sediments are more variable and range from 0.91 to 5.7. Only the three btrue contouriteQ cores (ENAM 96-03, 96-05 and 96-06) have focussing factors N 2.2. It seems that a sustained local higher mean sediment flux must combine with the short residence time of 230Thexcess in the water column to produce a regionally averaged sedimentation flux on the western margin of Rockall Basin that is higher than the singular value implied elsewhere in the NE Atlantic (Fig. 9). Acknowledgements We gratefully acknowledge the British Ocean Sediment Core Research Facility (BOSCORF) for part of the core material used in this study, and the NERC Radiocarbon Laboratory at East Kilbride (allocation 942.1201) and the NERC Uranium Series Facility at the Open University (allocation IP/791/1003) for the analyses undertaken for this work. The final version of this script benefited considerably from the reviews of the three journal referees. References [1] J.H. Chen, R.L. Edwards, G.J. Wasserburg, U-238, U-234 and Th-232 in seawater, Earth Planet. Sci. Lett. 80 (1986) 241 – 251. [2] D. Delanghe, E. Bard, B. Hamelin, New TIMS constraints on the uranium-238 and uranium-234 in seawaters from the main ocean basins and the Mediterranean Sea, Mar. Chem. 80 (2002) 79 – 93. [3] L.F. Robinson, N.S. Belshaw, G.M. Henderson, U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas, Geochim. Cosmochim. Acta 68 (2004) 1777 – 1789. [4] R.F. Anderson, M.P. Bacon, P.G. Brewer, Removal of Th-230 and Pa-231 from the open ocean, Earth Planet. Sci. Lett. 62 (1983) 7 – 23. [5] R.F. Anderson, M.P. Bacon, P.G. Brewer, Removal of Th-230 and Pa-231 at ocean margins, Earth Planet. Sci. Lett. 66 (1983) 73 – 90. [6] M.P. Bacon, Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic Ocean estimated from Th-230 measurements, Isot. Geosci. 2 (1984) 97 – 111. [7] S.B. Moran, M.A. Charette, J.A. Hoff, R.L. Edwards, W.M. Landing, Distribution of Th-230 in the Labrador Sea and its relation to ventilation, Earth Planet. Sci. Lett. 150 (1997) 151 – 160.

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