Testing portable luminescence reader signals against ...

Testing portable luminescence reader signals against OSL ages of coastal and desert dunefield sand in Israel Preliminary results Joel

Dorit

1 Sivan ,

Revital

3 Bookman ,

Naomi

4 Porat ,

Gloria I.

5 López

of Maritime Civilizations, Charney School of Marine Studies & the Leon Recanati Institute for Maritime Studies (RIMS), University of Haifa, Haifa, Israel; 2 The School of Sciences, Achva Academic College, Israel; 3 Dept. of Marine GeoSciences, Charney School of Marine Studies, University of Haifa, Israel; 4 Geological Survey of Israel, Jerusalem; 5 Luminescence Dating Laboratory, CENIEH, Burgos, Spain

INTRODUCTION & GOALS

Coastal Sand with OSL ages < 1.5 ka (bulk >150 µm)

a

 A Pulsed Photon-Stimulated Luminescence Unit (PPSL), (known as a Portable OSL reader) (Fig. 1), enables measurements of luminescence signals of quartz grains in small, natural and untreated sediment samples.

1.60

1.40

OSL age (ka years ago) 1.20 1.00 0.80 0.60 0.40

2) Coastal sand sheets and dunefields along the Mediterranean coast - formed since the middle Holocene (6 ka) and comprised of sand derived from the Nile Littoral Cell (Figs. 4-7).  PPSL measurements were done on the untreated >150 mm fraction left over from the sample (fraction) used for OSL dating. Two ~5 g sub-samples were measured (under subdued lighting conditions), and their signal averaged.

8

2

0 0

60,000

80,000

100,000

600,000 800,000 1,000,000 1,200,000 1,400,000 1,600,000

R² = 0.681

Fig. 1. The SUERC PPSL unit set up at the Luminescence Dating Laboratory, Geological Survey of Israel. (August 2016; G.I. López)

R² = 0.258

2,000,000

Fig. 5. Best fit of age groups of coastal sand between previously measured OSL ages, and PPSL values: a. 1.5 ka – Linear regression.

 PPSL signals differ by several orders of magnitude between modern and late Pleistocene sediments (Figs. 3-4).

NE

SW

a

 The coastal and Negev desert sand have different ratios between the OSL ages and the PPSL signals.

a

 Coastal sand show better correlations between PPSL signals and OSL ages than Negev samples (Figs. 4-5).

OSL age: 0.06±0.04 ka

 Exponential regression fits the data better than a linear one for interpolating ages for each dunefield as samples with different ages exhibit similar PPSL signals (Figs. 3-5).

South Coastal Dune Sections

 The coastal dune profiles yielded very low PPSL values representing young ages (Fig. 6).

OSL age (ka years ago) 15 10

5

0

10

0

8

OSL age (ka years ago) 6 4

800,000 1,000,000 1,200,000 1,400,000

2

0

200,000

1,800,000 2,000,000

Fig. 3. Exponential regression between previously measured OSL ages and PPSL values for the same subset of samples from the northwestern Negev dunefield. The dunefield does not have sand dating between 10-2 ka.

5,000

6,000

0

0

400,000 600,000 800,000 1,000,000 1,200,000 1,400,000

1,600,000 R² = 0.6489

b

Northeastern edge of Nizzanim dunefield (Fig. 7)

2

400,000 600,000

1,000

Av Net OSL (Photon Counts) 2,000 3,000 4,000

1

200,000

 11 untreated samples from 3 southern coastal (Nizzanim and Ashdod dunefields) transverse dune profiles were measured to investigate PPSL signal changes over depth (Figs. 6-7).

0

Coastal Sand (bulk >150 µm)

Northwestern Negev sand 20

1,800,000

120,000

RESULTS

25

1,600,000

R² = 0.845

3 4

Central Ashdod dunefield

5

Eastern edge of Ashdod dunefield

6

OSL age: 0.06±0.04 ka 7

Fig. 6. a. Semi-active transverse dune in central Ashdod dunefield (Fig. 2). b. Initial samples from southern coast dune sections showing the depth-dependent and relatively low PPSL signal values (Figs. 6a, 7). For better stratigraphical assessments, more samples are needed throughout each of the profiles.

1,800,000 2,000,000

Fig. 4. Exponential regression between previously measured OSL ages and PPSL values for the same subset of samples from the Mediterranean Coast dunefield. See enlargement of age groups in fig. 5.

SUMMARY OF ON-GOING RESEARCH  This study demonstrates that rapid, but very rough estimates of OSL ages may be achieved across different dunefields, mainly based on exponential regressions between PPSL values and existing OSL ages.  The spatial variability of the OSL-dated samples may contribute to the non-linear behavior. Fig. 2. The coastal and northwestern Negev dunefields of Israel. Their sand originates from the Nile River system but along different paths.

Av Net OSL (Photon Counts)

40,000

400,000

Depth (m)

1) The northwestern Negev desert dunefield - deposited since the LGM, and reworked during the late Holocene (Fig. 3).

10

Av Net OSL (Photon Counts)

a

Av Net OSL (Photon Counts)

 25 OSL-dated random sand samples from archives of two quartzdominated sand systems in Israel (Fig. 2) were studied:

0.00 0

20,000

 Therefore PPSL signals can be used as a proxy for relative age, can target field sampling for later full OSL dating, and can prioritize OSL dating in the laboratory.

METHODS

0.20

OSL age (ka years ago) 6 4

200,000

 The measurement does not take into account differences in dose rates which are crucial for absolute optically stimulated luminescence (OSL) age calculations.

 This study investigates the utility of PPSL signals in obtaining relative OSL age estimates by interpolating newly generated PPSL values of aeolian quartz sand samples from many sites within 2 Nilotic-sourced dunefields, that have been dated by OSL.

Coastal Sand with OSL ages > 1.5 ka (bulk >150 µm)

b

Av Net OSL (Photon Counts)

1 Dept.

1,2,3 Roskin ,

 Several variables can affect PPSL values of untreated bulk samples (more than for OSL age determination): first and foremost the dose rates, but also mineral provenance, sin- and postdepositional processes (e.g. re-working, bioturbation), grain coatings, etc.

Fig. 7. Recently stabilized transverse dune at northeastern edge of Nizzanim dunefield, southern coast of Israel.