Steve P. Lund

Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch

In the present-day climate, surface water salinities are low in the western tropical Pacific Ocean and increase towards the eastern part of the basin. The salinity of surface waters in the tropical Pacific Ocean is thought to be controlled by a combination of atmospheric convection, precipitation, evaporation and ocean dynamics, and on interannual timescales significant variability is associated with the El Niño/Southern Oscillation cycles. However, little is known about the variability of the coupled ocean–atmosphere system on timescales of centuries to millennia. Here we combine oxygen isotope and Mg/Ca data from foraminifers retrieved from three sediment cores in the western tropical Pacific Ocean to reconstruct Holocene sea surface temperatures and salinities in the region. We find a decrease in sea surface temperatures of ∼0.5 °C over the past 10,000 yr, whereas sea surface salinities decreased by ∼1.5 practical salinity units. Our data imply either that the Pacific basin as a whole has become progressively less salty or that the present salinity gradient along the Equator has developed relatively recently.

Climatic and Hydrologic Oscillations in the Owens Lake Basin and Adjacent Sierra Nevada, California

Oxygen isotope and total inorganic carbon values of cored sediments from the Owens Lake basin, California, indicate that Owens Lake overflowed most of the time between 52,500 and 12,500 carbon-14 (14C) years before present (B.P.). Owens Lake desiccated during or after Heinrich event H1 and was hydrologically closed during Heinrich event H2. The magnetic susceptibility and organic carbon content of cored sediments indicate that about 19 Sierra Nevada glaciations occurred between 52,500 and 23,500 14C years B.P. Most of the glacial advances were accompanied by decreases in the amount of discharge reaching Owens Lake. Comparison of the timing of glaciation with the lithic record of North Atlantic core V23-81 indicates that the number of mountain glacial cycles and the number of North Atlantic lithic events were about equal between 39,000 and 23,500 14C years B.P.

A comparison of Holocene paleomagnetic secular variation records from North America

The comparison of nine Holocene (0–12,000 years B.P.) records of paleomagnetic field secular variation (PSV) obtained from across North America indicates that distinctive field features in inclination and declination often can be traced for more than 4000 km without significant change in pattern. Independent age determinations corroborate these correlations while also suggesting that westward (or eastward) drift is not significant. On the other hand, northward drift may be a significant process at least between 1500 and 5500 years B.P. Several methods of analyzing the overall PSV character point to circularity (vector looping) as a preferred means of defining coherent space-time vector variations which are termed “waveforms”. A few distinctive waveforms, each discernible for about 1400 years, dominate PSV across North America during the Holocene; 4.5 loops and 2 loops being associated with clockwise and counterclockwise circularity, respectively. The clockwise loops appear to be periodic, occurring every 2400 years, and most likely result from the recurrence of a distinctive regional dynamo source in the outer core. The pattern of circularity noted in North America during the last 3000 years correlates in time with similar circularity changes seen globally. This may indicate that there is some degree of coupling between different regional dynamo sources within the outer core. The fact that some North American waveforms are almost identical to waveforms observed elsewhere around the world at different times may indicate that similar distinctive regional dynamo sources recur randomly throughout the outer core.

Geomagnetic field excursions occurred often during the last million years

Scientists studying western North Atlantic Ocean deep-sea sediments have discovered that the Earths magnetic field underwent 14 local excursions since the last global magnetic-field polarity reversal 780,000 years ago. These excursions coincide with similar excursions identified elsewhere on the planet—leading to the conclusion that excursions are global in nature, occur a significant portion of the time, and are an integral part of geomagnetic field secular variation between reversals. Excursions are defined [Verosub and Banerjee, 1977] as anomalous magnetic field directions whose equivalent virtual geomagnetic poles (VGPs) are more than 45° away from the North Geographic Pole, while VGPs within 45° of the North Geographic Pole are considered to be typical secular variation. (VGPs are calculated from local magnetic field directions which locate the magneticfield North Pole by assuming that the directions are caused by a simple dipole or bar magnet situated at the center of the Earth.)

Age of the Mono Lake excursion and associated tephra

Abstract The Mono Lake excursion (MLE) is an important time marker that has been found in lake and marine sediments across much of the Northern Hemisphere. Dating of this event at its type locality, the Mono Basin of California, has yielded controversial results with the most recent effort concluding that the MLE may actually be the Laschamp excursion (Earth Planet. Sci. Lett. 197 (2002) 151). We show that a volcanic tephra (Ash ♯15) that occurs near the midpoint of the MLE has a date (not corrected for reservoir effect) of 28,620±300 14C yr BP (∼32,400 GISP2 yr BP) in the Pyramid Lake Basin of Nevada. Given the location of Ash ♯15 and the duration of the MLE in the Mono Basin, the event occurred between 31,500 and 33,300 GISP2 yr BP, an age range consistent with the position and age of the uppermost of two paleointensity minima in the NAPIS-75 stack that has been associated with the MLE (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009). The lower paleointensity minimum in the NAPIS-75 stack is considered to be the Laschamp excursion (Philos. Trans. R. Soc. London Ser. A 358 (2000) 1009).

Measurement of the degree of smoothing in sediment paleomagnetic secular variation records: an example from late Quaternary deep-sea sediments of the Bermuda Rise, western North Atlantic Ocean

Abstract Replicate paleomagnetic secular variation (PSV) records for the last 15,000 yr have been recovered from two deep-sea sediment gravity cores on the Bermuda Rise, western North Atlantic Ocean. The records have been correlated using several different parameters and dated by correlation to a nearby piston core which has a high-resolution 14 C chronology. The PSV records are systematically subdued with respect to the expected local magnetic field variability during the Holocene, when sedimentation rates were about 10 cm/kyr. During the Late Pleistocene, when sedimentation rates were a factor of 2–3 higher, the PSV variability was significantly larger. We attribute this to smoothing of the PSV records by a depositional/post-depositional remanence (DRM/PDRM) acquisition process with a 10–20 cm lock-in interval. The degree of PSV smoothing was evaluated by comparing the VGP angular dispersion and vector spectral content of the Bermuda Rise cores with high-resolution PSV records from North America and Great Britain. These results suggest a 50% reduction in the DRM/PDRM recording of vector variability during the Holocene on the Bermuda Rise, with most of the PSV reduction occurring for features of less than 2000 yr in duration. We can reproduce the subdued PSV pattern with a mathematical model that simulates the site DRM/PDRM remanence acquisition process. Finally, the replicate gravity cores document that PSV records may be reproducible on a local scale, even though they only retain a low-pass filtered record of true magnetic field variability.

Influence of the Pacific decadal oscillation on the climate of the Sierra Nevada, California and Nevada

Mono Lake sediments have recorded five major oscillations in the hydrologic balance between A.D. 1700 and 1941. These oscillations can be correlated with tree-ring-based oscillations in Sierra Nevada snowpack. Comparison of a tree-ring-based reconstruction of the Pacific Decadal Oscillation (PDO) index (D’Arrigo et al., 2001) with a coral-based reconstruction of Subtropical South Pacific sea-surface temperature (Linsley et al., 2000) indicates a high degree of correlation between the two records during the past 300 yr. This suggests that the PDO has been a pan-Pacific phenomena for at least the past few hundred years. Major oscillations in the hydrologic balance of the Sierra Nevada correspond to changes in the sign of the PDO with extreme droughts occurring during PDO maxima. Four droughts centered on A.D. 1710, 1770, 1850, and 1930 indicate PDO-related drought reoccurrence intervals ranging from 60 to 80 yr.

Geomagnetic record in Minnesota lake sediments—Absence of the Gothenburg and Erieau excursions

Several excursions of the geomagnetic field during the Brunhes epoch have been postulated on the basis of paleomagnetic data from sediments. It has further been suggested that these excursions may represent sudden fluctuations of the geomagnetic field, manifested especially in fluctuations of the local geomagnetic inclination. In this study we present high-resolution data of inclination variations recorded in the sediments of two Minnesota postglacial lakes. To the best of our knowledge, our data cover two overlapping time windows, 0 to 9600 B.P. and about 9000 to 16,000 B.P. The results strongly suggest that no excursions have occurred in Minnesota over the past 16,000 yr. Thus, it appears that the Gothenburg (Sweden) excursion at about 12,500 B.P. is not worldwide and that the regional Erieau (Lake Erie) excursion does not extend to Minnesota. We suggest that these two previously claimed excursions might not reflect geomagnetic field behavior but merely poor paleomagnetic “recording” within the sediments used for these studies.

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.