Stability of superparamagnetic minerals in soils and rocks Can frequency-dependent susceptibility be used as a proxy for climate in the past? Jan Igel1, Holger Preetz1,2 & Christian Rolf1 1Leibniz Institute for Applied Geophysics, Hannover, Germany Leibniz Institute for 2OFD Niedersachsen, Federal Competence Center for Soil and Applied Geophysics Groundwater Protection / UXO Clearance, Hannover, Germany

Motivation Results & Interpretation

I Frequency-dependent (FD) magnetic susceptibility is a property of I Samples treated with oxygen free water and samples treated superparamagnetic (SP) minerals. The presence of these minerals with sodium sulphide (shown here) show similar results.

in soils and sediments is commonly attributed to neoformation Magnetic susceptibility during weathering and pedogenesis. 55000 Warm and humid conditions give rise to neoformation of SP mi- 50000 I Rocks: after dithionite 45000 reduction experiment Tuff nerals and thus frequency-dependent susceptibility is often used as treatment Tephra palaeoclimate proxy in sediment stratigraphy. Rhyolite SI] 40000 6 −

I SP minerals can also be of lithogenic origin and occur in magmatic [10 Soils: LF rocks and volcanic ashes. However, a lithogenic origin is commonly κ ≈ Luvisol Humous loess 10000 Laterite neglected in palaeoclimatic studies. Terra Rossa

I Pedogenic SP minerals are assumed to be less stable than lithoge- 5000 nic SP minerals. 0 0 50 100 150 200 250 time [days]

Materials I No decrease of κLF or ∆κ in reducing environment → SP minerals stable regardless of their origin (lith./ped.) A number of 7 samples with high I Some soils show increase of κLF and ∆κ during reduction content of SP minerals were in- → neoformation of SP minerals due to bacterial activity vestigated. The rocks had been Magnetic susceptibility 1.5 crushed and the soils homogeni- before treatment after reduction sed prior to the experiment. after dith. extraction κLF = κ @ 505 Hz

rocks soils Name Origin Parent κLF κFD Origin of κHF = κ @ 5005 Hz 1 material [10−6 SI] [%] magn. minerals

rocks LF, initial κ ∆κ = κ − κ

/ LF HF LF

Tuff Yukka Mtn, USA 12400 28.3 lithogenic κ Tephra Eifel, GER 6300 3.4 lithogenic 0.5 κLF − κHF Rhyolite Moamba, MOZ 3200 10.7 lithogenic κFD = soils κLF

Luvisol Moamba, MOZ Basalt 5600 8.1 ped./lith. 0

Humous loess Bavaria, GER Loess 510 7.4 pedogenic Tuff Tephra Rhyolite Luvisol Laterite Laterite Vogelsberg, GER Basalt 52000 6.0 ped./lith. Terra Rossa Humous loess Terra Rossa Obrovac, Croatia Limestone 2600 10.5 ped./lith.? Absolute frequency dependence 1.5 before treatment after reduction I κLF and ∆κ decrease Experimental design after dith. extraction after dithionite treat- rocks soils ment for all but one 1 I Samples have been stored in reductive conditions during 7 months sample. initial

(oxygen free water or sodium sulphide). ∆κ / I κFD decreases for soils After this period, iron was extracted by citrate-bicarbonite- ∆κ I 0.5 but remains constant dithionite. This treatment is commonly supposed to dissolve pe- for rocks. dogenic ferrimagnetic minerals, but to save lithogenic ones.

I Susceptibility was monitored during the whole experiment. 0 → Dithionite digests Tuff Tephra Rhyolite Luvisol Laterite all exposed ferrimagne- Terra Rossa Conclusions: Climate proxy? Yes, but . . . Humous loess tic minerals (lithogenic Relative frequency dependence

25 minerals at the surface before treatment after reduction of grains and pedogenic I SP minerals are stable under reducing conditions and thus may be after dith. extraction preserved or even enriched in sediments. 20 minerals) I Lithogenic input is often present in soils or sediments and has to be → Dithionite doesn’t taken into consideration when interpreting susceptibility as proxy 15 rocks soils digest the lithogenic mi- [%] FD κ nerals embedded in the for palaeoclimate. 10 rock matrix. I Dithionite digestion can be used to distinguish embedded litho- genic SP minerals (no change in κFD) from exposed pedogenic 5 (decrease of κFD). Materials with both, lithogenic and pedogenic SP minerals mixed, 0 I Tuff Tephra Rhyolite Luvisol Laterite are difficult to interpret. Terra Rossa Humous loess

DGG 2015 http://www.liag-hannover.de [email protected]