New Italian directional and intensity archaeomagnetic reference curves for the past 3000 years: Insights on secular variation and implications on dating

Summary Archaeomagnetic directions of one hundred and forty-one archaeological structures have been studied from 21 sites in Austria, 31 sites in Germany and one site in Switzerland. Characteristic remanent magnetisation directions obtained from alternating field and thermal demagnetisations provided 82 and 78 new or updated (12 and 10 per cent) directions of Austria and Germany, respectively. Nine of the directions are not reliable for certain reasons (e.g. displacement) while three of the features are not well dated. Apart from this some updated age information for the published databases is provided. Rock magnetic experiments revealed magnetite as main magnetic carrier of the remanences. The new data agree well with existing secular variation reference curves. The extended data set covers now the past 3500 years and a lot of progress were made to cover times BC with data. Here enhanced secular variation is observed manifested in declinations with values up to 70°. The new data will allow for recalculation of archaeomagnetic calibration curves for Central Europe from mid Bronze Age until today.

Download Full-text

Implications of Dipole Moment Secular Variation from 50,000–10,000 Years for the Radiocarbon Record

Radiocarbon ◽

10.1017/s0033822200013618 ◽

1992 ◽

Vol 34 (2) ◽

pp. 189-198 ◽

Cited By ~ 4

Author(s):

R. S. Sternberg ◽

P. E. Damon

Keyword(s):

Dipole Moment ◽

Production Function ◽

Secular Variation ◽

Good Match ◽

Average Value ◽

The Past ◽

Dc Gain ◽

Ramp Function ◽

Holocene Record ◽

Linear Ramp

Sparse paleointensity data from 10–50 ka suggest that the average dipole moment (DM) was 50–75% of the average of 8.67 μ 1022 A m2 for the past 5 Ma, and 8.75 μ 1022 for the past 12 ka. A linear ramp function, increasing the DM from 4 to 8.75 μ 1022 A m2 between 50–10 ka BP, generates a total 14C inventory of 126 dpm/cme2, agreeing very well with an inventory assay of 128 dpm/cme2, which includes 14C in sediments. With the Lingenfelter and Ramaty (1970) production function and a model DC gain of about 100, this DM function would give a Δ14C of 500‰ at 20 ka BP, consistent with the Barbados coral record, and also gives a good match to the Holocene record. A Laschamp geomagnetic event at about 45 ka BP, with a DM of 25% of its average value and lasting 5 ka, would only increase the present inventory by 0.3–1.2 dpm/cme2, and would probably have only a small effect on Δ14C at 20 ka BP, but could produce a short-lived 14C spike of over 500‰.

Download Full-text

A comparison of geomagnetic secular variation as recorded by historical, archaeomagnetic and palaeomagnetic measurements

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1982.0070 ◽

1982 ◽

Vol 306 (1492) ◽

pp. 103-112 ◽

Cited By ~ 15

Keyword(s):

Magnetic Field ◽

Lake Sediments ◽

Secular Variation ◽

Geomagnetic Field ◽

Secular Change ◽

Mathematical Approach ◽

Geomagnetic Secular Variation ◽

The Past ◽

Palaeomagnetic Data ◽

Field Behaviour

Palaeomagnetic methods can extend the documentary record of changes in the Earth’s magnetic field far into the past. Tolerable agreement is found between various methods, demonstrating the geophysical value of palaeomagnetic experiments. Combining results from the different approaches of investigating secular change can lead to a better perspective and to superior models of geomagnetic field behaviour. Lake sediments have recently been found to hold remarkably detailed signatures of past field changes. A mathematical approach to formulating an empirical description of global geomagnetic field behaviour is proposed and applied to palaeomagnetic data spanning the last 10 ka.

Download Full-text

Secular variation of the geomagnetic dipole during the past 2000 years

Geochemistry Geophysics Geosystems ◽

10.1029/2007gc001728 ◽

2008 ◽

Vol 9 (1) ◽

pp. n/a-n/a ◽

Cited By ~ 49

Author(s):

Jean-Pierre Valet ◽

Emilio Herrero-Bervera ◽

Jean-Louis LeMouël ◽

Guillaume Plenier

Keyword(s):

Secular Variation ◽

The Past ◽

Past 2000 Years ◽

Geomagnetic Dipole

Download Full-text

Review: the effects of secular variation in seawater Mg/Ca on marine biocalcification

Biogeosciences Discussions ◽

10.5194/bgd-6-7325-2009 ◽

2009 ◽

Vol 6 (4) ◽

pp. 7325-7452 ◽

Cited By ~ 4

Author(s):

J. B. Ries

Keyword(s):

Secular Variation ◽

Marine Organisms ◽

Coralline Algae ◽

Geologic History ◽

Ambient Seawater ◽

The Past ◽

Microbial Biofilms ◽

Tube Worms ◽

Probable Consequence ◽

Series Of Experiments

Abstract. Synchronized transitions in the polymorph mineralogy of the major reef-building and sediment-producing calcareous marine organisms and abiotic CaCO3 precipitates (ooids, marine cements) throughout Phanerozoic time is believed to have been caused by tectonically-induced variations in seawater molar Mg/Ca (>2="aragonite seas"; <2="calcite seas"). Here, I review a series of experiments in which extant calcifying taxa were reared in experimental seawater formulated over the range of mMg/Ca ratios (1.0 to 5.2) that occurred throughout their geologic history. Aragonite-secreting bryopsidalean algae and scleractinian corals and calcite-secreting coccolithophores exhibited higher rates of calcification and growth in the experimental seawaters that favored their skeletal mineral. These results support the assertion that seawater Mg/Ca played an important role in determining which hypercalcifying marine organisms were the major reef-builders and sediment-producers throughout Earth history. The observation that primary production increased along with calcification in mineralogically-favorable seawater is consistent with the hypothesis that calcification promotes photosynthesis within autotrophs through the liberation of CO2. The Mg/Ca ratio of calcite secreted by the coccolithophores, coralline algae and reef-dwelling animals (crustacea, urchins, calcareous tube worms) declined with reductions in seawater Mg/Ca. Calcifying microbial biofilms varied their mineral polymorph with seawater Mg/Ca (mMg/Ca<2=low Mg calc; mMg/Ca>2=arag+high Mg calc), suggesting a nearly abiotic mode of calcification. These results indicate that biomineralogical control can be partially overridden by ambient seawater Mg/Ca and suggests that modern high Mg calcite organisms probably secreted low Mg calcite in calcite seas of the past. Notably, Mg fractionation in autotrophic organisms was more strongly influenced by changes in seawater Mg/Ca, a probable consequence of them inducing a less controlled mode of calcification simply through the removal of CO2 via photosynthesis. This body of work also has implications for thermal-chemical reconstructions of seawater that are based upon skeletal Mg/Ca. And by identifying how marine calcifiers respond to changes in seawater Mg/Ca and absolute Ca2+ concentration, this work should enhance our interpretation of the parallel studies investigating the effects of CO2-induced ocean acidification on marine calcification.

Download Full-text