Faculty of Geosciences Department of Earth Sciences Paleomagnetic Laboratory Fort Hoofddijk

Faculty of Geosciences Department of Earth Sciences Paleomagnetic Laboratory Fort Hoofddijk

AF demagnetization reflects only one component (MT or HT)

All patterns

Prospectives AF fails

Zijderveld diagrams of M-type samples with varying MT:HT intensity ratios (left) with their corresponding sketched unblocking spectra (right). The individual unblocking spectra for the MT (blue) and LT/HT (red) components contribute to the total unblocking spec- trum (green), and explain the variation in NRM beha- vior in the Zijdervelds.

We correlate the normal base (N4) to Chron C4n.2n, and the top (N1) to C3Bn, so that the core yields ~1.5 Myr. Correlations of N4 to C4An, or N1 to C3An.1n are precluded by biostratigraphy (Congeria rhomboidea sublittoral mollusc biozone in N4, and

Prosodacnomya vutskitsi littoral mollusc biozone in N1) and seis- mic correlation.

Rock magnetic results

1 Paleomagnetic Laboratory Fort Hoofddijk, Dept. of Earth Sciences, Utrecht University, The Netherlands

2 Hungarian Natural History Museum, Dept. of Paleontology and Geology, Budapest & MTA-MTM-ELTE Research Group for Paleontology, Budapest, Hungary

Paleomagnetism in the Pannonian; Problems, Pitfalls and Progression

Nick Kelder ¹ , Karin Sant ¹ , Imre Magyar ² , Mark J. Dekkers ¹ , Wout Krijgsman ¹

The HT-polarities of the M-type samples are consistent with the directions of the S-type samples. We assume that they reflect an early-diagenetic signal and that their combined polarity pattern can be used for magne- tostratigraphic dating of the studied succession.

Terminology

M-type: multiple-polarity sample

S-type: (apparent) single polarity sample

LT/MT/HT: low/medium/high temperature range TH/AF: thermal/alternating field demagnetization

We have mapped pitfalls for paleomagnetism in

greigite-bearing sediments and exemplified a practical method to tackle dating problems in the Pannonian Basin, and beyond. We strongly wish for an improved geochemical characterization of different greigite forms in order to make more progression with paleomagnetic methods in greigite-prone basins.

Lake Pannon

study area in Hungary ^b

Study area

Dating of Upper Miocene sediments in the Pannonian Basin has proven difficult due to endemic biota, scarcity of reliable radioisotopic data, and generally inconsistent paleomagnetic results associa- ted with diagenetic forms of the iron sulfide greigite ^a . Here, we present a greigite-bearing well core from Hungary in which opposite magnetic polarities within the same horizons can be distinguis- hed. We demonstrate that magnetostratigraphic dating of such material is feasible by using a careful thermal demagnetization method with extra small steps (10 °C). Classic

alternating field demagnetization results in an unreliable polarity pattern and should be avoided.

Contact: n.a.kelder@uu.nl

References: ^a Babinszki, E., et al. (2007); PPP 252, 626–636 / ^b Magyar., I., et al. (2013); GPC 103, 168–173 / ^c Hilgen, F. J., et al. (2012); The Neogene Period.

Careful methodology

20 210

PJ141 _Up/W

N 240

290

300

310 Int

_i

=4.77mA/m

Int

₂₄₀

=6.24mA/m Int

₃₁₀

=7.01mA/m

TH

AF ^Int

AF=0

=7.22mA/m 0 15 30 45 60 90

HT component finished by AF demag

N PJ24A

230 20 250

310 330

Int

_i

=5.32mA/m 290 Int

₂₃₀

=4.96mA/m Int

₃₁₀

=1.95mA/m

TH

Up/W N

Int

_AF=0

=4.63mA/m 25 0

45 80

AF

0 20

35 50

80

Int

_AF=0

=11.509mA/m

Up/W N

AF

PJ140A

Int

_i

=11.88mA/m N

Int

₂₃₀

=4.73mA/m Int

₃₁₀

=12.42mA/m

20 180

230 250 290

310

360

250

310 120

360

TH

Left: A clear greigite sample. A) Irreversible decrease in magnetiza- tion ~250-400 °C depicts greigite break-down.

Pyrite to magnetite peak between 400 and 580 °C;

B/C) IRM values in the greigite range; D)

Slightly negative peak at 50-60 mT, closed large concentric contours, indi- cative of SD greigite.

Right: Noisy sample. E) As in A) but less pronounced, possibly mixture of primary greigite and magnetite;

G/H) Noisy IRM, values on boundary magnetite/ grei- gite. H) Shifted peak to lower coercivities at 20-40 mT. Closed converging

contours and large vertical spread. Indicative of a

greigite/magnetite mixture or greigite sample with a substantial SP grain popu- lation.

0 200 400 600

Temperature (

^o

C) 0

0.04 0.08

0 2.0 4.0 6.0 0 0.5 1.0 1.5 2.0 2.5

1 10 100 1000

1 10 100 1000

LAP LAP

GAP ^B

1/2

: 74.1mT DP: 0.14 B

_1/2

: 125.9mT DP: 0.36 B

_1/2

: 25.7mT

DP: 0.32 Gr adient IRM (10

-5

Am

2

/k g)

Applied Field (mT)

H u(mT)

-40 -20 20 40

0

H

C

(T)

0.0 0.04 0.08 0.12 Am

²

/kg T

²

SF 10

20 15 10 5 0 -5

A _PAN67

B

C

D F

G H

0.008 0.013 0.018 0.023 0.028

To ta l M ag ne tiz at io n (A m 2/ kg )

0 200 400 600

Temperature (

^o

C)

0 0.4 0.8 1.2 1.6

IRM (10

-6

Am

2

/k g)

0 1.0 2.0 3.0

4.0 1 10 100 1000

1 10 100 1000

GAP

Gr adient

B

_1/2

: 56.2mT DP: 0.25 B

_1/2

: 16.6mT

DP:0.40

B

1/2

:281.8mT DP: 0.40

Applied Field (mT)

Hu(mT)

-40 -20 20 40

0

0.0 0.04 0.08 0.12

H

_C

(T)

Am

²

/kg T

²

0.0 0.2 0.4 0.6

SF 15 0.8

E _PA256

H

0

50

100

150

200

250

300

350

400

450

500

550 -90 -60 -30 0 30 60 90

Inclination -90 -60 -30 30 90

HT and S-types

0 60

Inclination

n=108 n=74

-90 -60 -30 0 30 60 90 Inclination

n=43

?

-90 -60 -30 30 90

HT-component MT + HT range

0 60

Inclination

n=65

0

5 0

1 0 0

15 0

20 0

2 5 0

3 0 0

3 5 0

4 0 0

45 0

5 0 0

5 5 0 -90 -60 -30 30 90

MT-component

0 60

Inclination

n=60

M-type S-type Combined AF

S-type M-type

FORC FORC IRM Curie

IRM Curie

S-type

PJ257A Up/W

N

Int

_i

=5.36mA/m

20

180 230 270

310 360

A). PJ53 Up/W

N

Int

_i

=3.342mA/m 20 180

240 310 330 LT

HT MT

100 200 300 400

Demagnetization steps (°C)

Thermal decay diagrams Zijdervelds

In tensity (uA/m)

1k 2k 3k 4k 0

Net Effect: LT:R

B). PJ140A Up/W

Int

_i

=11.88mA/m Int

₂₃₀

=4.73mA/m Int

₃₁₀

=12.42mA/m

20 180

230 250 290

310

360

120

250

310

360

LT HT MT

100 200 300 400

In tensity (uA/m)

2k 6k 10k 14k 0

LT:N MT:R HT:N

Net Effect:

MT:C

Up/W

N C). PJ24A

230 20 250

310 330

Int

_i

=5.32mA/m 290 Int

₂₃₀

=4.96mA/m Int

₃₁₀

=1.95mA/m

LT MT

HT

100 200 300 400

In tensity (uA/m)

1k 3k 5k

0

Net Effect: LT:C MT:N HT:R

HT:R

Correlation to the GPTS

St ra t. heigh t (m)

Right: The HT compo- nent can be completely removed by consecutive TH-AF cleaning, avoiding the growth of magnetite from pyrite > ~400 °C.

Ma

Age/ Stage 6

6.5

7

7.5

8

8.5

9

9.5

Tortonian Messinian

C4A C4 C3B C3A

GPTS

^c

Combined

Polarity data

C3Bn

C4An

C4Ar.1n C3An.2n C3An.1n

C4n.2n C4n.1n

100

150

200

250

300

350

400

450

500

1E-08 1.E-07 1

.

E

-

06

Specific

Susceptibility

n=341 Core PAET-30

Stratigraphy

End rődi F m. A lg yő Fm. Újfalu Fm.

-90-60-30 30 90

Sed.

rate

29 cm/ky 39 cm/ky 33 cm/ky 59 cm/ky

0 60

Inclination

m

N4 N3 N2 N1

?

?

(m3/kg)

(°C)

(°C)

HT (LT) component Unblocking Spectrum MT component Unblocking spectrum Combined

Unblocking Spectrum HT (LT) component Unblocking Spectrum MT component Unblocking spectrum Combined

Unblocking Spectrum

Some samples show one clear direction

same level same level

M-type: conflicting polarities

Acknowledgement: We thank the Hungarian National Research, Development and Innovation Office (NKFIH 116618) for their financial support.

To ta l M ag ne tiz at io n (A m 2/ kg )

General: 10-20 °C steps are key to resolve the

different components.