Martha Schwartz

Environmental factors as complicating influences in the recovery of quantitative geomagnetic‐field paleointensity estimates from sediments

We have recently recovered replicate records of the Earths magnetic-field (relative) paleointensity for 12-71,000 years BP from marine sediments of the western North Atlantic Ocean. Our records are remarkably similar to two other recently published relative paleointensity records from marine and lacustrine sediments in Europe. This suggests to us that, even over this wide region, sediments may serve as reasonable, correlatable recorders of fluctuations in the strength of the Earths magnetic field. If this is true, then similar records from around the globe could eventually provide valuable information about the deep-Earth processes which create the field. We note, however, that our intensity record is significantly correlated with the down-core ratio of magnetic susceptibility (χ) to anhysteretic remanent magnetism (ARM). This ratio is a measure of the relative grain size of magnetite (the primary magnetic mineral in these sediments), and as such is a sediment magnetic property which is controlled only by the local depositional environment (and indirectly by global climate). We are concerned that the European records may be similarly biased by climatic or other environmental factors, possibly synchronous with ours. We caution that extreme care must be taken to understand and remove any such magnetic influences from sediment relative paleointensity records before they are used as quantitative estimators of the past intensity of the Earths magnetic field.

Early sediment diagenesis on the Blake/Bahama Outer Ridge, North Atlantic Ocean, and its effects on sediment magnetism

Sediment magnetic and geochemical studies of a suite of deep-sea sediment cores from the Blake/Bahama Outer Ridge (BBOR), North Atlantic Ocean, have identified two current redox boundaries in surficial (Holocene), carbonate-rich sediments over much of the BBOR. The upper Mn+4/Mn+2 redox boundary is associated with a spike in the concentration of solid-phase Mn (as MnO2); the lower Fe+3/Fe+2 redox boundary is associated with a spike in the concentration of solid-phase Fe (as goethite, αFeOOH). Over much of the BBOR, high sediment magnetic intensities occur in surficial, carbonate-rich sediments associated with these redox boundaries and lower intensities occur in deeper (late Pleistocene) carbonate-poor sediments. This relationship is opposite to that expected if sediment magnetism simply reflects the clastic (noncarbonate) sediment fraction. The surficial, high sediment magnetic intensities are due primarily to the following two factors: (1) magnetic mineral authigenesis associated with early diagenesis and (2) the presence of abundant <0.1 μm magnetite crystals interpreted to be bacterial magnetosomes. Magnetosomes are almost absent in the late Pleistocene low-carbonate sediments owing, most likely, to local Pleistocene environmental conditions (high clastic flux, low organic flux) which did not favor their growth. The sediment natural remanent magnetization is strongly correlated with the sediment clastic fraction and is relatively unaffected by early diagenesis and the presence of abundant bacterial magnetite. If this is typical, bacterial magnetite may be more abundant in nature but less important to sediment paleomagnetic records, than previously thought.

Geomagnetic field intensity from 71 to 12 ka as recorded in deep‐sea sediments of the Blake Outer Ridge, North Atlantic Ocean

In this paper we estimate relative geomagnetic field paleointensity between 71 and 12 kyrs BP as recorded in three deep sea sediment cores from the Blake Outer Ridge, western North Atlantic Ocean. Paleointensities were estimated by normalizing sediment natural remanant magnetization separately to (1) magnetic susceptibility, (2) anhysteretic remanant magnetization and (3) saturation isothermal remanent magnetization (SIRM). In one core, paleointensities were estimated within short time windows which display uniform sediment magnetic characteristics, and offsets between windows were removed to minimize environmental biases. We find that most features of our records are preserved regardless of normalizer choice, but we chose SIRM as the best normalizer for the final paleointensity estimates. Our three records preserve and agree upon a number of short-duration (∼103 years) paleointensity features even though the cores are separated by almost 250 km. We conclude that these are real geomagnetic field signals of at least local extent. We also identify a number of differences between our records which must be artifacts of sediment remanence acquisition or paleointensity normalization. Such artifacts occur as either (1) baseline shifts between time intervals with slightly different sediment magnetic characteristics or (2) differences in amplitude of short duration events. In spite of these environmental biases, the number and ages of relative paleointensity highs and lows are preserved. Thus sediment paleointensity estimates may be used locally for high-resolution chronostratigraphic correlation. Correlation of our paleointensity records with other records from the same extended region (North Atlantic Ocean-western Europe) indicates that major paleointensity features appear in all regional records. However, differences in the ages and disagreement in magnitude and number of individual features call into doubt the use of these relative paleointensity records for high-resolution chronostratigraphic correlation on a broader regional scale or for quantitative estimation of past geomagnetic field variability.

26Al, 10Be and U–Th isotopes in Blake Outer Ridge sediments: implications for past changes in boundary scavenging

We have investigated the distributions of 26 Al, 10 Be, U^Th isotopes, and stable 27 Al and 9 Be in core CH88-11P (30‡40PN, 74‡41PW; 3337 m) from the Blake Outer Ridge (BOR) in the western North Atlantic, an area that is characterized by high terrigenous input. Results show authigenic 26 Al/ 27 Al of 6 2^8U10 314 (atom/atom), authigenic 10 Be/ 9 Be of 0.5^2.6U10 38 (atom/atom), 6 0.2^1.8U10 6 atoms/g of 26 Al, and 5.3^15.1U10 8 atoms/g of 10 Be in the core over the last glacial/interglacial cycle. These values, as well as total 230 Thex/ 232 Th and 10 Be/ 9 Be ratios, were all at their minima during the last glacial maximum (LGM), reflecting intensified terrestrial influx to the area. Aluminosilicate material (clays) in this influx preferentially scavenged 230 Th over the two other particle-reactive nuclides, 26 Al and 10 Be, such that their boundary scavenging during LGM followed the order 230 Ths 10 Bes 26 Al, as opposed to 10 Bev 230 Ths 26 Al in the Holocene. They appear at variance with the order 10 Bes 26 AlW 230 Th as would be predicted from the relative particle reactivities of the three species. These scavenging characteristics point to 10 Be/ 26 Al as a potentially more suitable proxy for paleoproductivity than 10 Be/ 230 Thex. Mass-balance considerations for 26 Al and 10 Be show a five-fold LGM-to-Holocene increase in deep-water circulation in the study area, with little change in ocean productivity except during the deglaciation when it increased noticeably. fl 2001 Elsevier Science B.V. All rights reserved.