Holocene paleomagnetic secular variation records from the western Equatorial Pacific Ocean

Earth and Planetary Science Letters 246 (2006) 381 – 392 www.elsevier.com/locate/epsl

Holocene paleomagnetic secular variation records from the western Equatorial Pacific Ocean Steve P. Lund a,⁎, Lowell Stott a , Martha Schwartz a , Robert Thunell b , Amy Chen c a

Department of Earth Sciences, University of Southern California, Los Angeles, United States b Department of Geology, University of South Carolina, United States c Torrance High School, Torrance, CA, United States Received 12 July 2005; received in revised form 8 February 2006; accepted 17 March 2006 Available online 23 May 2006 Editor: V. Courtillot

Abstract We have carried out detailed rock magnetic and paleomagnetic studies on a suite of four deep-sea sediment cores from Indonesia and the Philippine Islands, which form a transect from 6.3°N to 10.6°S. The cores have average sediment accumulation rates of 35–65 cm/kyr that all yield high-resolution paleomagnetic secular variation (PSV) records. The PSV records contain 44 correlatable inclination and declination features that corroborate the individual PSV records and indicate that they are all continuous with no significant stratigraphic gaps. Thirty-four AMS radiocarbon dates provide a high-resolution time stratigraphic framework for the PSV records, which extend from ∼0–18,000 yrs BP. These are the first high-resolution Holocene PSV records ever recovered from anywhere on Earth within 17° of the Equator (almost 1/3 of the Earth's surface area). Statistical analysis of the Holocene PSV records indicates that they have ΔI anomalies of −5° to −9°, consistent with 5 MA averaged results. The strong similarity in ΔI anomaly pattern of Holocene and longer-term (5 Ma) averaged PSV data suggest that the dynamo process causing the anomaly is largely described by Holocene PSV. By contrast, the Holocene PSV records have VGP angular dispersions that are significantly lower than 5 MA averaged PSV data yet significantly higher than PSV data averaged over only the last few thousand years. This difference indicates that whatever dynamo process causes the overall directional variability of the Earth's magnetic field is not totally described by recent (last few thousand year) PSV or even by late Quaternary (last ∼20,000 yrs) PSV. © 2006 Elsevier B.V. All rights reserved. Keywords: paleomagnetic secular variation; paleomagnetism; geodynamo

1. Introduction Our knowledge of prehistoric geomagnetic field secular variation since the last magnetic polarity reversal (∼ 780,000 yrs BP) is very fragmentary due to the limited number of high-resolution paleomagnetic secu⁎ Corresponding author. E-mail address: [email protected] (S.P. Lund). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.03.056

lar variation (PSV) records available. There are no continuous Holocene (0–12,000 yrs BP) PSV records from anywhere on Earth within 10° of the Equator and only three PSV records from two localities (NE Australia, 17.3°S [1]; Hawaii, 19.7°N [2]) within 20° of the Equator (more than one-third of the Earth's surface area). This lack of spatial coverage limits our ability to determine the evolutionary space/time pattern of geomagnetic field secular variation and consider its

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Fig. 1. Map of the western Equatorial Pacific Ocean region showing the locations of deep-sea sediment piston cores studied in this paper.

implications for the core dynamo process that generates the main field. This paper presents new PSV results from a group of four deep-sea sediment piston cores collected from Indonesia and the Philippine Islands region (Fig. 1) that form a transect from 6.3°N to 10.6°S latitude. Three of the cores contain complete Holocene sediment sequences with average accumulation rates of 40– 65 cm/kyr, which preserve very high-resolution PSV records. The resulting PSV records contain a reproducible record of Holocene geomagnetic field variability that adds significantly to our understanding of lowlatitude field variability. 2. Core descriptions and sampling In 1998, the RV Marion Dufresne collected an extensive set of deep-sea sediment piston cores from the western Equatorial Pacific Ocean as part of the IMAGES paleoclimate initiative. Four of those cores (Table 1) were sampled for this paleomagnetic study: MD982162 (hereafter referred to as MD62), MD982170 (MD70), MD982177 (MD77), and MD982181 (MD81). MD70 was the only piston core to have a short gravity Table 1 Deep-sea piston core site locations Core

Latitude

Longitude

Water depth (m)

Core length (cm)

MD982162 MD982170 MD982177 MD982181

4°41.33′S 10°35.52′S 1°24.2′N 6°30′N

117°54.17′E 125°23.26′E 119°04.68′E 125°49.2′E

1855 830 968 2114

5530 3526 3214 3650

core (MD70G) collected with it as a trigger core. All five piston and gravity cores were cut into ∼ 150 cm sections, split in half on shipboard, and visually described. All five cores are composed of fine-grained, brown to grey hemi-pelagic sediments that are uniform in appearance. The working halves of all cores are stored at the Oregon State University core repository. The uppermost 900 cm of MD62, 450 cm of MD70, 700 cm of MD77, and 1500 cm of MD81 were identified as containing Holocene and Holocene/Pleistocene transition sediments based on preliminary oxygen isotope stratigraphy (Stott and Thunell unpublished data) and sampled for this study. Cores MD70G (0–120 cm) and MD62 (0– 900 cm) and the uppermost sections of MD70 (0– 150 cm) and MD81 (0–150 cm) were sampled discretely using a square-cross section mini corer [3] to recover one 2 × 2 × 2 cm sediment cube per horizon. The bottom half of one section from core MD62, which extended from 225–300 cm, was not sampled for it appeared that the sediments had been distorted during transport. MD70G, MD70, and MD81 (0–150 cm) were sampled at a 2.5 cm interval; MD62 was sampled at a 5 cm interval. All remaining sediment sections were sampled using u-channels to collect ∼ 150-cm-long, continuous sediment columns with a 2 × 2 cm square cross section. 3. Paleomagnetic and rock magnetic measurements The initial NRMs of all discrete samples were measured using a 2G cryogenic magnetometer. At least two contiguous samples per meter in each core

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were then demagnetized in alternating magnetic fields (af) up to 100 mT at 10 mT steps and remeasured after each demagnetization step. All samples lost a soft component of magnetization with somewhat random directions by 20 mT and thereafter demagnetized in a straight line toward the origin (Fig. 2a). All other samples were then demagnetized at 20 mT and 40 mT and the 20 mT NRM results were used for the PSV analysis. U-channel measurements followed a similar pattern except that the NRMs of all samples were initially measured and then step-wise af demagnetized and remeasured at 10 mT steps up to 100 mT using an automated 2G u-channel cryogenic magnetometer. The u-channel NRM demagnetization pattern (Fig. 2b–d) was the same as that for discrete samples with simple af demagnetization in a straight line toward the origin from 20 mT to 100 mT. Here, too, 20 mT NRM results were used for the PSV analysis. Rock magnetic studies started with bulk magnetic susceptibility measurements of all discrete samples using a discrete-sample sensor attached to a Bartington MS-2 susceptibility meter. The magnetic susceptibility of all u-channels was measured at a 2-cm interval using a 7-cm diameter long-core sensor attached to the Bartington meter. After NRM demagnetization, all discrete samples and u-channels were then given an anhysteretic remanence (ARM) by applying a 0.05 mT dc field superposed on a 100 mT af field. The ARMs of all discrete samples and u-channels were measured and then step-wise af demagnetized and remeasured at

Fig. 2. Zijdereldt diagrams illustrating the NRM demagnetization pattern of four selected sample horizons from the studied cores. Solid (open) symbols track the horizontal (vertical) component of the initial NRM and NRM after step-wise af demagnetization in 10 mT steps up to 100 mT.

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Fig. 3. Rock magnetic characteristics of core MD62. The grey area indicates the transition from relatively strong magnetic intensities above to very weak magnetic intensities below, which we interpret to be a paleoredox boundary.

20 mT and 40 mT. Finally, a saturation isothermal remanence (SIRM) was given to all samples using a 1 T dc field. The SIRMs were then af demagnetized at remeasured at 20 mT and 40 mT. The rock magnetic data for cores MD62, MD70 (NRM and Chi only), MD77, and MD81 are summarized in Figs. 3–6. MD62, MD70, and MD77 all contain

Fig. 4. Selected rock magnetic characteristics of core MD70. The grey area indicates the transition from relatively strong magnetic intensities above to very weak magnetic intensities below, which we interpret to be the current redox boundary.

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Fig. 5. Rock magnetic characteristics of core MD77. The grey area indicates the transition from relatively strong magnetic intensities above to very weak magnetic intensities below, which we interpret to be a paleoredox boundary.

evidence for a significant change in rock magnetic properties 1 to 9 m below the sediment/water interface. The rock magnetic properties of all of the older sediments are typically an order of magnitude lower in intensity than those of the younger sediments. The boundaries between stronger and weaker intensities are very abrupt in MD62 (860–890 cm, Fig. 3) and MD77 (660–680 cm, Fig. 5) and occur in Pleistocene-aged sediment (see below). By contrast, the transition in MD70 is much less abrupt (130–210 cm) and occurs in Holocene-aged sediments. We interpret the transition in MD70 to reflect a current chemical redox boundary with oxic conditions above and sulfate reduction and significant magnetic mineral dissolution below. The older boundaries in MD62 and MD77 probably reflect paleoredox boundaries, which terminated when significant magnetic mineral dissolution stopped in late Pleistocene time as organic flux decreased significantly or clastic (Fe) flux increased significantly. Our paleomagnetic records are limited to portions of the cores that do not show significant evidence for magnetic mineral dissolution.

0° and then making minor shifts between adjacent core segments based on comparisons with the other independent cores. Cores MD70G and MD70 were correlated using both paleomagnetic and rock magnetic features. The two cores have moderately different sedimentation rates in their uppermost 1.5 m, even though they were collected within meters of one another. It is possible that one core was compressed during coring relative to the other core, but there is no evidence for sediment disruption in the correlatable magnetic features. The paleomagnetic results for MD70 and MD70G are plotted versus the MD70 depth scale in Fig. 8. As will be discussed below, both cores appear to be missing the last ∼ 2000 yrs of sediment. The PSV records of all five cores were initially correlated using their known oxygen isotope stratigraphies and 34 AMS radiocarbon dates (Table 2, see also Figs. 7–10). On the basis of these initial correlations, we were able to identify 21 inclination features and 23 declination features, which are distinctive and correlatable among the five independent PSV records. Those features are numbered in Figs. 7–10. These 44 correlatable features are consistent stratigraphically with the independent isotope and radiocarbon dating; the features are also consistent among themselves with proper inclination/declination phase relationships preserved and with no significant features missing (or extra) in any one record. The correlatable paleomagnetic features have been used to develop stratigraphic transfer functions between cores MD62, MD70, MD77, and MD81. The depth– depth transfer functions for MD62 and MD77 relative to MD81 are shown in Fig. 11; MD81 is used as the ‘standard’ reference core because it has the highest average sediment accumulation rate of all studied cores.

4. Paleomagnetic secular variation records The inclination and declination records for cores MD62, MD70–MD70G, MD77, and MD81 are displayed in Figs. 7–10. The data are NRM values after 20mT af demagnetization. The declination records for each of the cores have been reconstructed by rotating the average declination of each ∼150-cm core segment to

Fig. 6. Rock magnetic characteristics of core MD81. This core does not show any significant evidence for magnetic mineral dissolution associated with current or paleo redox boundaries.

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385

Fig. 7. Paleomagnetic results (20 mT NRMs) from core MD62. Selected inclination and declination features are numbered, which can be correlated with the other independent PSV records from the study area. Calibrated AMS radiocarbon dates from this core are shown at top.

The transfer functions generally indicate smoothly proportional and nearly constant sediment accumulation among the three long records. The smoothly continuous transfer functions between cores and presence of all significant directional features in all cores both suggest that no single core has marked discontinuities in sedimentation (> 10–20 cm) relative to the other cores. The MD70 PSV record is much shorter than the other three PSV records (as discussed below) and we are less confident of its correlation to the other three records. Our analysis below focuses on the three entire Holocene records from MD62, MD77, and MD81. 5. Chronostratigraphy

Fig. 8. Paleomagnetic results (20 mT NRMs) from cores MD70 (open symbols) and MD70G (closed symbols). Selected inclination and declination features are numbered, which can be correlated with the other independent PSV records from the study area. Calibrated AMS radiocarbon dates from this core are shown at top.

34 AMS radiocarbon dates, listed in Table 2, have been recovered from the Holocene and Latest Pleistocene parts of cores MD62, MD70, MD70G, MD77, and MD81. These dates have been used to develop individual chronologies for all of the cores. All of the dates have been calibrated using the Calib 5.0 program of Stuiver et al. [4] and reservoir corrections of Broecker et al. [5]. The median calibrated ages and the lower and upper limits of permissable ages are listed in Table 2; the calibrated ages are used below to develop individual core chronologies. 28 radiocarbon dates for cores MD62, MD77, and MD81 are plotted in Fig. 12 using the MD81 stratigraphy and transfer functions from Fig. 11. Four radiocarbon dates from cores MD70 and MD70G were

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Fig. 9. Paleomagnetic results (20 mT NRMs) from core MD77. Selected inclination and declination features are numbered, which can be correlated with the other independent PSV records from the study area. Calibrated AMS radiocarbon dates from this core are shown at top.

not used for this analysis because the PSV correlations from those cores to MD81 were less constrained than we deemed appropriate. Similarly, the radiocarbon dates at 992 cm in MD62 and 12 cm in MD77 are not plotted in

Fig. 12 because they are located beyond the interval of correlatable PSV data and are therefore not well constrained in their possible stratigraphic correlations to MD81.

Fig. 10. Paleomagnetic results (20 mT NRMs) from core MD81. Selected inclination and declination features are numbered, which can be correlated with the other independent PSV records from the study area. Calibrated AMS radiocarbon dates from this core are shown at top.

S.P. Lund et al. / Earth and Planetary Science Letters 246 (2006) 381–392 Table 2 AMS radiocarbon dates Depth (cm) MD-62 11 (38) b 141 (220) c 341 (455) 521 (755) 661 (886) 698 (942) 861 (1205) 992

C14 yrs BP

±1σ

Cal. yrs BP a

(Lower– median–upper)

910 2330 3620 6970 9540 10,590 14,890 17,740

40 40 40 40 40 40 40 50

289 1626 3215 7246 9814 11,092 16,471 19,605

450 1851 3426 7395 10,109 11,497 16,980 20,258

592 2064 3633 7553 10,324 11,963 17,509 20,923

MD-70 200 300

6900 9710

75 70

7317 10,284

7390 10,458

7452 10,599

MD-70G 7 102

2370 8520

60 75

1900 8879

1971 8976

2036 9043

MD-77 12 109 (222) 212 (425) 412 (825) 512 (990)

395 1870 3760 9470 12,300

90 110 35 55 50

1050 3386 9795 13,174

0 1345 3586 10,050 13,545

1637 3809 10,313 13,845

MD-81 12 55 99 145 238 401 501 711 755 861 941 961 1011 1086 1211 1286 1486

580 815 1010 1090 1900 3960 4070 6950 7360 9480 11,050 13,700 13,650 13,400 15,550 16,800 20,300

110 200 100 120 110 150 190 50 50 65 310 200 100 55 65 95 260

0 0 331 392 1166 3455 3496 7266 7617 9804 11,319 14,403 14,957 14,299 17,206 18,553 22,414

149 352 534 594 1373 3849 3998 7378 7730 10,056 12,286 15,527 15,491 14,938 17,741 19,179 23,175

338 629 679 834 1597 4254 4499 7476 7849 10,309 13,024 16,164 15,991 15,649 18,304 19,830 23,999

a

Calibrated radiocarbon dates were calculated using the Calib 5.0 program of Stuiver et al. [4] and reservoir corrections of Broecker et al. [5]. All samples measured at the Woods Hole Radiocarbon Lab. b MD-81 equivalent sediment depths are in parentheses. c Dates in italics were not used in age reconstructions.

The 28 radiocarbon dates in Fig. 12 generally follow a smooth trend from 0 to ∼ 23,000 Cal yrs BP. But, four of the dates, indicated by arrows in Fig. 12, have age ranges that are too old relative to the remaining dates. Three of these dates come from MD81 and one from MD62. We presume these dates are contaminated by

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anomalously old carbon and have removed them from further consideration. The pair of anomalous old dates near 10 m in MD81 raise some possibility that this is a contaminated interval (slump?) in MD81. However, the PSV within this interval is distinctive and correlatable with cores MD62 and MD77, and a date of 13,545 Cal. yrs BP from MD77 falls right on the expected trend of all other dates near 10 m in Fig. 12, based on the PSV correlations and transfer function in Fig. 11. Therefore, we think that the MD81 dates near 10 m are anomalous but the sediment interval itself and PSV record are reasonable. The solid curve in Fig. 12 is our preferred chronology for MD81. The curve was developed using only the MD81 radiocarbon dates and smoothly goes through the allowable age ranges of all MD81 dates. The curve also goes through the allowable age ranges of all other dates from MD62 and MD77 that have been transferred to the MD81 stratigraphy. This corroborates both the MD81 radiocarbon dates and the PSV correlations noted above. We can also use the radiocarbon dates in Table 2 and depth–depth correlations of Fig. 11 to develop chronologies for MD62 and MD77. Fig. 13 plots all seven MD62 radiocarbon dates and 14 dates from MD77 and MD81 that could be correlated using the PSV. The dates follow a smooth progression and we have drawn a simple smooth curve composed of five linear segments though all the data. This curve goes through all of the MD62 permissible age ranges and most of the MD77 and MD81 age ranges as well. Fig. 13 is our preferred chronology for MD62. Fig. 14 plots all five of the MD77 radiocarbon dates together with 16 dates from MD77 and MD81 that could be correlated using the PSV. These dates also follow a smooth progression and we have drawn a simple smooth curve composed of five linear segments though all these data as well. This curve goes through three of the MD77 permissible age ranges and most of the MD62 and MD81 age ranges. But, two of the MD77 dates (3586, 10050) appear to be biased to old age relative to other nearby dates from MD62 and MD81; these dates are also biased to old age relative to the derived chronologies for MD81 and MD62. We have tried to draw interval line fits that best reflect all of the plotted dates in each interval. Fig. 14 is our preferred chronology for MD77. Finally, we have developed a tentative chronology for MD70 and MD70G. We previously used the PSV, rock magnetic, and limited oxygen isotope data from cores MD70 and MD70G to develop a depth–depth correlation between MD70 and MD70G, which permitted us to plot the PSV data in Fig. 8. We have used that same correlation to plot the four MD70 and MD70G

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Fig. 11. Depth–depth transfer functions for cores MD62 and MD77 relative to MD81 (the master core for all stratigraphic correlations). The correlations are based on the numbered inclination and declination features noted in Figs. 7–10.

Fig. 13. Time–depth plot for core MD62. See text for more detailed discussion.

radiocarbon dates versus depth in MD70 in Fig. 15; we have added three age estimates for deeper parts of MD70 based on oxygen isotope features, which were AMS radiocarbon dated in other cores. We have also added three radiocarbon dates from cores MD62, MD77, and MD81 that we have tentatively correlated

using the PSV records in Fig. 8. All seven of the radiocarbon dates can be fit by a single straight line in Fig. 15 with an intercept of about 1700 Cal. yrs BP. The intercept suggests that cores MD70 and MD70G are both missing the last ∼ 1500–2000 yrs of sediment. We have no clear explanation for the apparent sediment loss, but it is most likely due to a local erosional discontinuity. The straight-line curve in Fig. 15 is our

Fig. 12. Time–depth plot for core MD81. Arrows indicate individual radiocarbon dates that are anomalously old and presumed to be contaminated with ancient carbon. See text for more detailed discussion.

Fig. 14. Time–depth plot for core MD77. See text for more detailed discussion.

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Fig. 15. Time–depth plot for cores MD70 and MD70G. See text for more detailed discussion.

preferred chronology for the Holocene part of MD70 and MD70G.

Fig. 16. Inclination time series for the three most complete PSV records recovered in this study. Numbered correlatable inclination features match those noted in Figs. 7–10.

6. PSV analysis The final inclination and declination records for MD62, MD70, and MD81 are plotted versus time in Figs. 16 and 17. The numbered inclination and declination features from Figs. 7–10 are shown as well. It is clear that the three records share the same general PSV pattern with all numbered features being present in all three cores and having ages that are not significantly different. One complication is that MD77 has inclination and declination amplitudes that are only half that of the two other cores, which bracket it spatially. We think that the diminished PSV amplitudes in MD77 are due to increased smoothing of the PSV record due to relatively low sediment accumulation rate (∼ 40 cm/kyr versus ∼ 50 cm/kyr and ∼ 65 cm/kyr for MD62 and MD81, respectively) and shallower water depth (∼ 900 m versus ∼ 1900 m and ∼ 2100 m for MD62 and MD81, respectively), which probably produce a thicker interval of sediment bioturbation and smoothing at the site of MD77. But, we assume that the smoothing process was unbiased, such that averaged directions from MD77 properly reflect ambient average field variability at the site. Our new PSV records can be used to assess the general characteristics of low-latitude geomagnetic field variability by statistical comparison with PSV from other parts of the World. Two statistical parameters most

often measured in PSV studies are the ΔI anomaly, which is the site mean inclination minus the expected axial dipole field inclination, and the angular dispersion

Fig. 17. Declination time series for the three most complete PSV records recovered in this study. Numbered correlatable declination features match those noted in Figs. 7–10.

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associated with a site's vector (or equivalent VGP) variability. The global pattern of the ΔI anomaly estimates how well the axial-dipole hypothesis, the cornerstone of plate tectonic reconstructions, fits the actual spatial pattern of geomagnetic field behavior. The global pattern of angular dispersion should provide some measure of the spatial pattern of intrinsic ‘energy’ or ‘dynamics’ in the core dynamo process. This variability may be due to differing proportions of dipole versus non-dipole field variability [6] or it could be interpreted as the relative importance of primary versus secondary family spherical harmonic components [7], two orthogonal components of the geomagnetic field that may have fundamental relationships to dynamo theory [8]. Table 3 summarizes the ΔI anomaly and VGP angular dispersion in our four new Equatorial PSV records. The ΔI anomaly was perhaps first quantified by Wilson [9] who noticed that the average paleomagnetic pole positions associated with individual geographic regions (e.g., Australia, Europe, North America) were always farther from the sampled region than the known geographic pole. This offset, termed the far-sided effect, is due to paleomagnetic inclinations that are systematically more negative than their axial dipole expectation (negative ΔI anomaly), on average. McElhinny et al. [10] have determined the global ΔI anomaly for the last 5 Ma (Fig. 18, solid circles), on average, and noted that the ΔI anomaly is mostly negative with a maximum anomaly near the Equator and a latitudinal variation that is symmetric about the Equator and zonal (any site along a line of latitude will have the same magnitude of ΔI anomaly). Their analysis also indicates that the ΔI anomaly has persisted with the same general pattern and magnitude for the last 5 million yrs during both normal and reversed polarity. But, Johnson and Constable [11] have argued that non-zonal components of the ΔI anomaly are significant and have persisted for the last several million years, with some Equatorial regions having more significant negative ΔI anomalies than other regions and some high-latitude regions actually having positive ΔI anomalies (see also [12]).

Fig. 18. ΔI anomaly as a function of latitude for selected time intervals: solid symbols—last 5 Ma average; open symbols—last few thousand years average; open squares—last ∼ 10–20,000 yrs average. New results from this study are labeled for clarity.

Holocene PSV records provide an important shorterterm view of the ΔI anomaly. PSV records for the last several thousand years ([13], PSVMOD1.0 time series downloadable at http://earth.usc.edu/~slund), shown by open circles in Fig. 18, display the same general ΔI anomaly pattern as the 5-Ma averages (although they are missing data from Equatorial latitudes), but they do suggest the presence of positive ΔI anomalies near 50° latitude in both hemispheres. Previously published PSV data for the last ∼ 30,000 yrs [2,12] and our new Holocene results from Equatorial latitudes, shown by open squares in Fig. 18, also replicate the general ΔI anomaly pattern of the 5-Ma averages. Moreover, they also corroborate the positive ΔI anomaly results near 50°N noted above and they fill in the short-term statistical data gap for Equatorial latitudes. (The positive ΔI anomalies in Fig. 18 (open symbols) come from sites on multiple continents in both hemispheres. They occur near 40°S in Australia and South America and near

Table 3 PSV statistical parameters Core

Latitude

Interval (ka)

AD Inc

Site Inc

α95

ΔI anomaly

VGP dispersion

MD62 MD70 MD70G MD77 MD81

− 4.7° − 10.6° − 10.6° 1.4° 6.5°

0–18 2–6 2–6 0–17 0–17

− 9.3° − 20.5° − 20.5° 2.8° 12.8°

− 15.1° − 27.2° − 27.6° − 1.8° 3.9°

1.5° 1.7° 1.0° 0.9° 0.6°

−5.8° −6.7° −6.9° −4.6° −8.9°

8.6° 5.9° 4.1° – 6.8°

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50°N in East Asia, North America, and Europe [12,13].) The strong similarity between the short-term (103 and 104 yr ave) and long-term (106 yr ave) ΔI anomaly results suggests that whatever causes the ΔI anomaly pattern is a dynamo process that is active today and totally represented in the PSV behavior of the last few thousand years and overall Holocene. Given the apparent common appearance of positive ΔI anomalies near 50°N and 40°S, we would hypothesize that highlatitude flux lobes [14], which occur historically [14,15] and in the paleomagnetic record of the last few thousand years [13], are the primary source for the high latitude positive ΔI anomalies. It is also likely that their global pattern [14,16] may well be the source for the significant negative ΔI anomalies at low latitude as well. In this view, the overall variability of high-latitude flux lobes (and their overall outer core structure) is largely described by PSV of the past few thousand years and has persisted with both normal and reversed polarity for millions of years. It is also important to note that the ΔI anomaly in our new PSV records cannot be caused by inclination errors (anomalous shallowing of paleomagnetic field directions) in sediment sequences. MD81 and MD77 are sediment sequences in the Northern Hemisphere that have shallower than expected inclinations, which produce negative ΔI anomalies; while, MD70 and MD62 are sediment sequences in the Southern Hemisphere that have steeper than expected inclinations, which produce negative ΔI anomalies. If sediment shallowing was significant in all four of these sediment sequences, we would see negative ΔI anomalies in the Northern Hemisphere and positive ΔI anomalies in the Southern Hemisphere. Analysis of PSVangular dispersions (either directions or their equivalent virtual geomagnetic poles, VGPs) has established that angular dispersion also displays a distinctive zonal pattern of amplitude variation with latitude. The average latitudinal pattern of VGP angular dispersion for the last 5 Ma [17] is shown in Fig. 19 (closed circles). (We use and prefer the form of VGP angular dispersion that is calculated relative to the mean VGP of the data distribution. An alternative form of the VGP angular dispersion [6] is calculated relative to the geographic North Pole. This adds a bias to the latitudinal pattern of VGP angular dispersion due to the latitudinal dependence of the ΔI anomaly.) The smallest variability in VGP angular dispersion occurs within 30° of the Equator and then increases smoothly with increasing latitude up to at least 70°. PSV records for the last several thousand years [13], shown by open circles in Fig. 19, display a similar overall pattern of VGP angular

391

Fig. 19. VGP angular dispersion as a function of latitude for selected time intervals: solid symbols—last 5 Ma average; open symbols—last few thousand years average; open squares—last ∼ 10–20,000 yrs average. New results from this study are labeled for clarity.

dispersion variability but with significantly lower values when compared with the 5-Ma averages. (The exceptionally low variability of three European sites in Fig. 19 is notable, but not easily explained.) Previously published PSV data for the last ∼ 30,000 yrs [2,12] and our new Holocene results from Equatorial latitudes, shown by open squares in Fig. 19, also display a similar overall pattern of VGP angular dispersion but with values that are significantly lower than the 5-Ma average VGP angular dispersion yet significantly higher than the data for the last few thousand years. A variety of parametric models, summarized in [6], have been developed to attempt to explain the observed spatial pattern of angular dispersion (biased by the ΔI anomaly) in an ad hoc manner. More recently, McFadden et al. [7] were able to relate the long term average pattern of VGP angular dispersion to historical field observations and developed model G, based on the relative importance of primary versus secondary family magnetic field components [8], to explain the latitude dependence. Whatever the source of the observed latitude-dependent VGP angular dispersion, it is clear that the source variability is not adequately described by recent (last few thousand year) PSV or even by known late Quaternary (last ∼ 30,000 yrs) PSV. If the global pattern of VGP angular dispersion provides some measure of the spatial pattern of intrinsic ‘energy’ or ‘dynamics’ in the core dynamo process, then the total range of energy or dynamics is only seen on timescales ≫ 20,000 yrs.

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7. Conclusions

References

We have carried out detailed rock magnetic and paleomagnetic studies on a suite of four deep-sea sediment cores (MD62, MD70, MD77, MD81) from Indonesia and the Philippine Islands, which form a transect from 6.3°N to 10.6°S. The cores have average sediment accumulation rates of 35–65 cm/kyr that all yield high-resolution PSV records. The four PSV records contain 21 correlatable inclination features and 23 correlatable declination features that corroborate the individual PSV records and indicate that they are all continuous with no significant stratigraphic gaps. Thirty-four AMS radiocarbon dates provide a highresolution time stratigraphic framework for the PSV records. The final PSV records for MD62, MD77, and MD81 extend from ∼ 0–18,000 yrs BP, while the PSV record for MD70 only extends from 2–6000 yrs BP. These are the first high-resolution paleomagnetic records of Holocene PSV ever recovered from anywhere on Earth within 17° of the Equator (almost 1/3 of the Earth's surface area). Statistical analysis of the four PSV records indicates that they have ΔI anomalies of − 5° to − 9°, consistent with previously published results. These results cannot be due to sediment inclination shallowing because the two Southern Hemisphere PSV records (MD62, MD70) have negative ΔI anomalies due to average inclinations steeper than axial dipole expectation. A strong similarity in ΔI anomaly pattern of Holocene and longer term (5 Ma) averaged PSV data suggest that the dynamo process causing the anomaly is largely described by Holocene PSV. By contrast, the four PSV records have VGP angular dispersions that are significantly lower than 5 MA averaged PSV data yet significantly higher than PSV data only averaged over the last few thousand years. This difference indicates that whatever dynamo process causes the directional variability of the Earth's magnetic field is not totally described by recent (last few thousand year) PSV or even by late Quaternary (last ∼ 30,000 yrs) PSV.

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