Atlantic Carbonate-Rich Sediment

The ²³⁵U- and ²³⁸U-decay-series in seawater produce a

231Pa_xs/²³⁰Th_xs-activity ratio of 0.093 (red dashed line in diagram). The diagram shows that bulk sediments and particle classes do not reflect this ratio. Bulk sediments, clay fractions and slowly settling fractions exceed this ratio by a factor of up to four, whereas the fast sinking particles carry an isotope ratio much lower than the production ratio.

231Pa_xs/²³⁰Th_xs-activity ratio

0 4 8 12

230Th_xs [dpm g^-1]

0 0.5 1 1.5 2

231 Pa xs [dpm g-1 ]

production ratio P ^{231 Pa/230 Th}

= 0.093

clay

2-20µm + slowly settling

fast settling bulk

Distribution of ²³⁰ Th _xs and ²³¹ Pa _xs in sediment particle classes of opal-rich and carbonate-rich sediments

Atlantic Carbonate-Rich Sediment

The sediment at the higher focusing location (GeoB1027-2, light green bars) contains more fine material (clay and fine silt) than at the lower focusing location (GeoB1028-4, dark green bars).

The higher focusing location contains more ²³⁰Th_xs in the bulk sediment and in

all particles >2µm than the lower focusing location.

Possible explanation: The higher focusing location is 450 m deeper than the other core location, so that it receives 20 % more ²³⁰Th_xs by the particles that sink vertically through the water column.

>125 µm

0 2 4 6 8 10 12

230Th_xs [dpm g^-1]

Ψ=2.2 (G eoB1027-2) Ψ=1.7 (G eoB1028-4) bulk

<2 µm 2-20 µm 20-63 µm 63-125 µm

>125 µm bulk

<2 µm 2-20 µm 20-63 µm 63-125 µm

>125 µm

particle fraction

0 10 20 30 40 50

size fraction contribution [%]

The contribution of ²³⁰Th_xs and ²³¹Pa_xs in percentage delivered by each size fraction is calculated by multiplication of the specific activity with the size fraction contribution.

Clay contributes 82 % to the total ²³⁰Th_xs. In contrast only 63 % of ²³¹Pa_xs is contained in the clay fraction. Substantial contribution to ²³¹Pa_xs inventories is made by fine silt and slowly sinking particles (diatom shells).

230Th_xs

<2µm (82%) 2-20µm

(15%)

>20µm (3%)

231Pa_xs

<2µm (63%) 2-20µm

(28%)

>20µm (9%)

Contribution by each size fraction in PS1768-8

In Carbonate-rich sediments not only clay but also silt-sized and sand-sized particles (forams and coccoliths) are relevant for

230Th_xs-inventory, because they account for almost half of the total inventory. The

230Th_xs-inventory of the lower focusing site is more strongly supported by the larger (>20µm) particles (33%) than the higher focusing site (24%).

<2µm (53%) 2-20µm

(23%)

>20µm (24%)

GeoB1027-2

GeoB1028-4

<2µm (55%) 2-20µm

(12%)

>20µm (33%) 230Th_xs contribution by each size fraction

Reference

Geibert and Usbeck (2004): Adsorption of thorium and protactinium onto different particle types: Experimental findings. Geochimica et Cosmochimica Acta, 68(7): 1489-1501

Southern Ocean Opal-Rich Sediment

The grain size distribution in opal-rich sediments shows that clay (<2µm) and fine silt (2-20µm) form the two major fractions (each class ~40%). Sediment at the „focusing location“ (PS1768-8, light blue bars) contains slightly more fine material and less coarse particles than at the

„non-focusing location“ (PS1769-1, dark blue bars).

In particle fractions larger than 2µm ²³¹Pa_xs is adsorbed preferentially to fine silt and slowly settling diatoms and less

strongly to fast sinking particles, while ²³⁰Th_xs does not show any significant preference.

The clay fractions hold the highest specific activities of

230Th_xs and ²³¹Pa_xs. The focusing location receives more clay with lower ²³⁰Th_xs activity than the non-focusing location, where clay contains higher specific ²³⁰Th_xs activity.

However, both cores hold nearly the same amount of

230Th_xs in the clay fraction (82% and 79%, respectively).

fast settling

2-20 µm

slowly settling

0 10 20 30 40 50

size fraction contribution [%]

bulk

< 2 µm 2-20 µm

20-63 µm 63-125 µm

>125 µm

20-63 µm 63-125 µm

>125 µm bulk

< 2 µm 2-20 µm

20-63 µm 63-125 µm

>125 µm

20-63 µm 63-125 µm

>125 µm

particle fraction

Ψ=2.2 (PS1768-8) Ψ=1.0 (PS1769-1)

0 2 4 6 8 10 12

230Th_xs [dpm g^-1] 0 0.4 0.8 1.2 1.6 2

231Pa_xs [dpm g^-1]

EGU 2009

Pacific Carbonate-Rich Sediment

Sediment from the Panama Basin exhibits a similar grain size dependent ²³⁰Th_xs distribution as the Atlantic carbonate-rich sediment. The diagramm shows the contributions of grain size classes to total

230Th_xs inventories expressed as focusing factors Ψ calculated for the intervals

core-top to 9.5 kyr, 9.5 to 13.4 kyr, and 13.4 to 21 kyr. Calculations are based on bulk densities multiplied by relative mass contributions (in %) of individual size classes. In both cores 80 % of the sediment focusing can be attributed to the smallest (<20µm) grain size fraction.

Grain size specific focusing factors

0 2 4 6 8 10 12 14 16 18 20 22

age (kyr)

0 1 2 3 4 5 6 7 8 9 10 11

Ψ

0 2 4 6 8 10 12 14 16 18 20 22

age (kyr)

0 1 2 3 4 5 6 7 8 9 10 11

Ψ

Y69-71P ME0005-24JC

>125 µm 63-125 µm 20-63 µm <20 µm

Lateral sediment transport can have a sorting effect on particles. More fine material is accumulated at locations of higher focusing factors.

Generally, ²³⁰Th_xs is adsorbed mostly onto clay and fine silt. However, we find different distribution patterns in opal sediments and carbonate sediments.

Different particle size classes contribute different amounts of ²³⁰Th_xs and ²³¹Pa_xs to the total inventory. This needs to be considered when using the sedimentary ²³¹Pa_xs/²³⁰Th_xs-ratio from sites that are strongly affected by sediment redistribution.

Conclusions Introduction

In many paleoceanographic studies ²³⁰Th_excess-normalization and the ²³¹Pa_excess/²³⁰Th_excess-ratio are used as tools for the reconstruction of particle fluxes and ocean circulation.

230Th_xs and ²³¹Pa_xs analyses are commonly performed on bulk sediment samples. However, it is conceivable that these two particle-reactive radioisotopes are not equally distributed between the different sedimentary components, because Th and Pa show preferential adsorption to specific particle types (Geibert and Usbeck, 2004). Therefore we performed particle size specific analyses of ²³⁰Th_xs and ²³¹Pa_xs on deep sea sediment samples from three locations with contrasting sediment characteristics.

Location

The deep sea sediment samples were retrieved from the subtropical Atlantic and the tropical Pacific (both carbonate-rich sediments) and from the Southern Ocean (opal-rich sediments).

At each location two sediment cores differing in their focusing factors were sampled (see maps).

Method

Bulk sediments were fractionated into distinct particle size classes by sieving and settling. The size classes of the opal-rich Southern Ocean sediments was further split into density classes by settling (slowly and fast sinking particles).

sediment water focusing core depth factor

Y69-71P 2740 m 3-6 ME0005-24JC 2941 m 4-10 GeoB1027-2 2668 m 2.2 GeoB1028-4 2215 m 1.7 PS1768-8 3299 m 2.2 PS1769-1 3269 m 1.0

-20˚ -15˚ -10˚

-10˚

-5˚

-5˚ 0˚0˚

5˚

5˚

10˚

10˚

15˚

15˚

20˚ 25˚ 30˚

-50˚ 30˚

-50˚

-45˚

-45˚

-40˚

-40˚

-35˚

-35˚

-30˚

-30˚

-25˚

-25˚

-20˚

-20˚

-15˚

0 200 400km

Southern Ocean

Namibia

South Africa Walvis R

idge

Shona Ridge

Atlantic Ocean

-95˚

-95˚

-90˚

-90˚

-85˚

-85˚

-80˚

-80˚

-75˚

-75˚

-5˚ -5˚

0˚ 0˚

5˚ 5˚

10˚ 10˚

0 200 400 km

Galapagos

Colon Balboa

Quito

Cocos Ridge

2920m 2330m

Carnegie Ridge

Panama Basin

Pacific

Atlantic

S. Kretschmer, S. Kusch, W. Geibert, M. Rutgers van der Loeff, G. Mollenhauer

Alfred-Wegener-Institute, Bremerhaven, Germany sven.kretschmer@awi.de

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