The impact of ocean deoxygenation on iron release from continental margin sediments

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1 The impact of ocean deoxygenation on iron release from continental margin sediments Florian Scholz*, James McManus, Alan C. Mix, Christian Hensen and Ralph R. Schneider *Author to whom correspondence should be addressed, e mail: fscholz@coas.oregonstate.edu Radiocarbon dating and age model The age model for the sediment section from 0 to 345 cm is based on six radiocarbon dates. As foraminifera or other calcareous remains are scarce, radiocarbon measurements were carried out on organic matter 1,2. Prior to analysis samples were pre treated according to the common acid alkali acid extraction protocol 3. This procedure yields the alkali residue fraction and the humic acid fraction, both of which were subjected to radiocarbon measurements at the AMS facility of the Leibniz Laboratory for Radiometric Dating and Isotope Research at Kiel University. Any terrigenous carbon that is present in the sample is typically recovered in the alkali residue which is why the radiocarbon date of the humic acid fraction (always the younger one) was used for the age model (Supplementary Table 2). Radiocarbon dates were calibrated using CALIB version (ref. 4). In addition to the global ocean reservoir age correction of about 400 years that is subtracted by CALIB, we included an additional regional reservoir age correction of 400 years 1,2,5. The 14 C concentration of the sample at 431 cm was smaller than the 2 SD uncertainty of the radiocarbon measurement. The resulting age is thus likely to be a minimum age and was not explicitly included in the age model. Previous paleoceanographic studies on the Peruvian margin have demonstrated that shifts in the strength and position of the undercurrent as well as differences in organic matter preservation lead to coarser grain sizes and lower organic carbon concentrations during times of cooler temperatures and lower sea level 6,7. Following this rationale, tie points for the age model below 345 cm were selected by correlating maxima in grain size (as inferred from zirconium to rubidium ratios) and minima in organic carbon with the global benthic δ 18 O stack by Lisiecki and Raymo (ref. 8). The final age versus depth model is shown in Supplementary Figure 2. The close match between the δ 15 N records of M77/ and CD38 02 from the Nazca Ridge 9 (Supplementary Figure 3) are supportive of the assumptions used in deriving the age model. Furthermore, the age model of M77/ is in general agreement with those of ODP Sites 680 ( S, W, ~253 m water depth 6 ), 1228 ( S, W, ~273 m water depth 5,10 ), 681 ( S, W, ~150 m water depth 11 ) and 1229 ( S, W, ~150 m water depth 10 ) which are located within a distance of ~7 km and ~13 km from M77/ An erosional unconformity, covering parts of Marine Isotope Stage (MIS) 2 and 3 (~50 20 kyr BP), was identified at 423 cm. The sediments corresponding to this time interval were likely eroded during the time of lowest sea level at the Last Glacial Maximum (cp. ODP Sites 1228 and 1229; ref. 10). Redox related versus other influences on the concentration of iron relative to aluminum and titanium Most of the aluminum (Al), titanium (Ti) and iron (Fe) in continental margin sediments is supplied from terrigenous sources through river runoff or aeolian transport. While Al and Ti behave largely conservative during early diagenesis, a fraction of the terrigenous Fe (the reactive Fe pool, chiefly Fe (oxyhydr)oxides) may be subject NATURE GEOSCIENCE 1

2 to reductive dissolution and diffusive or advective transport across the sediment bottom water interface. If the Fe lost from the sediment is transported elsewhere within the water column, the sediment left behind will be depleted in reactive Fe relative to Al or Ti 12,13. Provided that the amount of Fe lost is sufficiently high compared to the overall terrigenous Fe, Al and Ti supply, Fe depletion may be illustrated by comparing its Fe/Al or Fe/Ti with the lithogenic background ratios or by calculating Fe XS (Fe XS = Fe T (Fe/Al) lithogenic x Al T ) 14. Most of the Al and unreactive Fe in continental margin sediments reside in alumosilicate minerals, whereas Ti is often contained in heavy minerals such as rutile. Given their common occurrence, it is generally preferable to normalize Fe to Al rather than Ti and to calculate Fe XS relative to (Fe/Al) lithogenic. However, because of the insufficient x ray yield for Al, the Fe data from XRF core scanning were normalized to Ti instead of Al. A prerequisite for the comparability of Al based Fe XS and Fe/Ti is that any downcore variability in Ti/Al, e.g., due to changes in sediment provenance 15, biological productivity 16 or grain size 17, is negligible relative to the redox related variability in Fe XS or Fe/Ti. Consistent with this prerequisite, Ti and Al are linearly correlated and Ti/Al ranges close to the ratio of the average upper continental crust 18 (Supplementary Figure 4). Moreover, Fe depleted samples show a similar relative offset from Al and Ti in both data sets (Supplementary Figure 4). Importantly, if current related grain size effects (rather than redox controlled Fe release) were an important reason for the observed downcore variability in Fe XS, Fe depleted samples would be also depleted in Al relative to Ti, as grain size effects are known to enrich Ti relative to both Fe and Al 17. Supplementary Table 1 Accuracy of the sediment digestion protocol. Measured values are given as mean ± SD. SDO 1 19 RR9702A 42MC 20 This study (n = 9) Certified This study (n = 6) Previous studies Al (wt.%) 6.20 ± ± ± ± 0.11 Ti (µg g 1 ) 404 ± ± ± ± 19 Fe (wt.%) 6.39 ± ± ± ± 7 Mo (µg g 1 ) 152 ± ± ± ± 7 U (µg g 1 ) 45.0 ± ± ± ± 0.22 Supplementary Table 2 Radiocarbon ages. Calibration data from ref. 21. Alkali residue fraction Humic acid fraction Depth 14 C age ± SD Cal age (1 SD range) Relative area 14 C age ± SD Cal age (1 SD range) Relative area Cal age used (1 SD range) b (cm) (yr BP) (yr BP) under distribution (yr BP) (yr BP) under distribution (yr BP) ± ± ± ± ± ± ± ± ± ± ± ± a ± 1580 a a Minimum age. b Age used includes an additional reservoir age correction of 400 yr 1,2,5. 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION Supplementary Figure 1 Bathymetric map of the Peruvian continental margin. Stars depict the location of M77/ , CD38 02 (ref. 9) and ODP Site 861 (ref. 10). Black circles depict the location of short sediment cores (multicores, MUCs) that are used for proxy calibration. Supplementary Figure 2 Age versus depth model for piston core M77/ NATURE GEOSCIENCE 3

4 15 N ( Air) CD38-02 M77/ Age (cal kyr BP) Supplementary Figure 3 δ 15 N records of M77/ and CD38 02 (ref. 9). The original age model of CD38 02 is based on oxygen isotope stratigraphy (Figure 3). Ti (wt.%) Fe (wt.%) r = Al (wt.%) a c Al (wt.%) Fe (XRF counts s -1 ) Fe (wt.%) Ti (wt.%) b d Ti (XRF counts s -1 ) Andesite in the Andean Arc Upper continental crust Supplementary Figure 4 Comparison of Fe, Al and Ti data from sediment digestion and XRF core scanning. a, Ti versus Al (digestion). b, Fe versus Ti (digestion). c, Fe versus Al (digestion). d, Fe versus Ti (XRF scanning). 4 NATURE GEOSCIENCE

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