A link between geomagnetic reversals and events and glaciations

EPSL Earth and Planetary Science Letters 147 (1997) 55-67

A link between geomagnetic reversals and events and glaciations

Horst-Uhich Worm *

Insitutjiir Geophysik, Uniuersitiit Giittingen. Herzberger Lmdstrasse 180, 37075 Giittingen, Germany

Received 16 September 1996; accepted 2 January 1997

Abstract

The apparent duration of geomagnetic polarity events in Arctic Ocean sediments is much longer than in sediments from lower latitudes. In fact, while the remanence of Brunhes age sediment cores from the Yermak Plateau at 82”N is fully reversed for * 30% of their lengths [l], the events often evade detection in many other continuously deposited sediments. For example, the Laschamp event is absent in an otherwise high-resolution record of secular variation from Lac du Bouchet [2], which is located near the Laschamp volcanics, where the event was first detected. Very short event durations of a few hundred years at the most have been suggested before [2,3]. Because sedimentation rates in the Arctic Ocean were increased during glaciations, the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals. This suggestion is supported by previous magnetostratigraphic results obtained from thick loess/paleosol sequences in China 141. These demonstrate that all polarity boundaries separating chrons and subchrons since the Gauss-Matuyama field reversal have been recorded in loess, and thus during periods of cold climate, although conflicting evidence exists for some boundaries. Furthermore, the ages of 22 events and chron boundaries have been compared with the oxygen-isotope record [5], thought to represent global ice volume. All events and reversals younger than 2.6 Ma may have occurred during periods of global cooling or during cold stages; however, some ages are still too poorly dated for a definite correlation. Climatic signals also exist in the two longest relative paleointensity records [6,7] but these are suspected to be caused by climatically driven variations in the rock magnetic parameters. A mechanism for field reversals may be the acceleration of the Earth’s rotation, caused by lowering of the sea level during glaciations. The short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed,

Keywords: magnetic field; reversals; paleomagnetism; glaciation; Milankovitch theory

1. Introduction

The geomagnetic polarity time scale (GPTS) has

been undergoing successive refinements ever since the first publication of a GPTS [8] but the polarity pattern for the Pliocene and Pleistocene was established about 30 years ago [9] and has remained

* E-mail: [email protected]

approximately unchanged, except for a general aging of most polarity boundaries due to systematically too young ages from K/Ar dating. The accuracy of the

recent GPTS [lo,1 11, however, has greatly improved, owing to astronomically derived calibration [5,12,13] and the 40Ar/ 39Ar dating method [lo]. Besides major field reversals separating chrons and subchrons, numerous short polarity events and excursions have occurred (e.g., [3]) whose existence and nature have always been a matter of controversial debate, not so

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56 H.-U. Worm/Earth and Planetary Science Letters 147 (1997) 55-67

much because of a lack of evidence for events but because there are many more sedimentary records that do not show indications of anything other than normal field polarity within, for example, the Brun- hes chron.

Marine sedimentation rates at high latitudes are highly variable and strongly climate dependent with particularly high sedimentation rates during glaciations. The contradictory evidence about the existence and duration of polarity events could thus be resolved if the events were very short in duration and occurred during glaciations. An attempt had therefore ben made to constrain the ages of the events by tying them to intensity lows in the relative paleointensity records. However, the only two relative paleointensity records spanning more than the last 400 kyr do not correlate and, moreover, show clear climatic signatures themselves. Although a climatic influence on the intensity of the Earth’s field cannot be ruled out at present, it seems more likely that the applied normalization procedures do not suppress climatically driven rock magnetic variations. Nonetheless, the comparison of ages of events and chron boundaries with the climatic record indicates that field reversals may be linked to glaciations.

2. Chrons, subchrons, events, and excursions

Polarity intervals are termed polarity chrons or subchrons depending on their duration. Polarity chrons lasted approximately 0. l- 1 Myr and polarity subchrons lo-100 kyr, according to the IAGA Sub- commission on the Magnetic Polarity Time Scale [14]. Global reversals shorter than 10 kyr are not termed subchrons but excursions, based on the argu- ment that, in order to be called subchrons, the geomagnetic field should have remained locked into the reversed direction for a measurable length of time [14] and, since the field reversal itself was thought to last about 5 kyr [ 14,151, the minimum duration of a subchron had to be N 10 kyr. Deviations of the virtual geomagnetic pole (VGP) from the geographi- cal pole by more than 40” are termed excursions as well [3,14]. Sometimes the terms polarity episode, reversal event, reversal excursion, or cryptochron are also used for short polarity intervals. In this paper the term “event” is used as the more general term for subchrons and excursions.

2.1. Duration of a field transition

Most estimates of the duration of field transitions have been derived from sediments. Sequences of lava flows that record a transition cannot be dated with the necessary precision to determine its length. Early estimates on the duration of a transition were l-10 kyr [15], which is on the order of the theoreti- cal free decay time of the dipole part of the geomagnetic field [ 161. It was probably for this reason that a duration of a few thousand years for a polarity transition was rarely questioned for decades. How- ever, the few marine sediments that record, for example, the Matuyama-Brunhes boundary, have high enough sedimentation rates to allow the resolution of shorter than N 1 kyr field changes. The paleomagnetic signal in marine sediments is, moreover, always smoothed by a finite width of lock-in depths, viscous overprints, and bioturbation, so that even a step-like field change would appear as a gradual transition. Despite this, reports on very rapid field transitions lasting less than 50 years [ 17,181 appear not to be generally accepted. However, a more recent high-resolution study on sediments with a sediment accumulation rate @AR) of 47 mm/kyr recovered from the North Pacific Ocean shows that the Matuyama-Brunhes directional transition occurred within 13 mm (i.e., 280 years [19]). The numerous reports on longer lasting transitions are mostly based on studies of more slowly deposited sediments, or the degree of smoothing by a wide lock-in depth interval may have been underestimated, particularly in sediments with intense bioturbation. Any study of the length of polarity transition derived from sediments can only give upper limits for the duration. Bearing this in mind, the evidence presented by Okada and Niitsuma [ 181 that the Matuyama to Brunhes directional transition lasted only 38 years should seriously be considered.

2.2. Geomagnetic events

Reports on events within the Brunhes and Matuyama chrons are numerous [3]; however, the nature and length of these events (whether they were excursions or subchrons) has been unresolved and has become even more enigmatic due to some recent, apparently contradictory, paleomagnetic studies.

H.-U. Worm/ Earth and Planetary Science Letters 147 (1997) 55-67 57

A discussion on the reality of events as global occurrences of subchrons is not just a question of nomenclature: it is important for the evaluation of the numerous relative paleointensity results obtained

from sediments. To the authors knowledge, none of the published relative paleointensity records were

obtained on sediments that show any directional indication for subchrons within the Brunhes. If there

had been global field reversals lasting for several

thousand years this would imply that those sediments

maintained a field record that is much more smeared

in time than commonly assumed.

Champion et al. [20] compiled the then available evidence for events during the last 1 Myr and sug-

gested that eight events in the Brunhes chron should

be termed subchrons. More recently, Jacobs [3] discussed in detail the evidence for events and their

nature. Nowaczyk et al. [l] list ten events with ages up to 500 ka that are documented for at least two

well separated sites. However, reservations against

the acceptance of subchrons during the Brunhes chron are widespread among geomagnetists, and the recent polarity time scales [ 10,l l] present the Brunhes chron

as a “black box” of uninterrupted normal polarity,

except for a cryptochron dated at 493-504 ka in

[ill.

3. Marine sedimentary records of events

The evidence for events being fully reversed fields

and estimates for their duration appear to decrease with site latitude. Sediment cores from the Yermak Plateau (at 82”N) exhibit several fully reversed inter-

vals with durations that are apparently many times longer than the polarity transitions [l]. Based on

AMS- I4 C age determinations, S l8 0 measurements, and ‘30 Th/ “Be stratigraphy, an age span of Recent to 170 ka has been deduced for the two cores under study. Astonishingly, core PS 2213, up to a depth of

4 m, dated at 130 ka, is for more than 50% (!> of reverse polarity. The authors estimate that the four events within the last 130 kyr have lasted between 5

and 11 kyr. It has to be pointed out, however, that the 14C dates indicate highly variable sedimentation in the upper meter and that the S “0 curve allows no unambiguous determination of the duration of the reverse intervals. Nonetheless, the sediments were undoubtedly deposited during the Brunhes chron.

Similarly high proportions of reverse inclinations

have been determined for sediments younger than 60 ka from the eastern Arctic Ocean at 80.5”N [21].

Sedimentary records from the Norwegian-Green-

land Sea at 69”N [22] and the V#ring Plateau at 66”N [23] also contain several events, only a few of which

are of fully reverse direction, and, in first order, the narrower the interval, the less reverse the direction.

Assuming a constant SAR of 10 cm/kyr throughout

the Brunhes chron, Bleil [23] estimated a maximum

duration of 3-5 kyr for the events.

Sedimentation rates at ODP Site 884 in the North

Pacific Ocean at 51”N are still high, with about 5

cm/kyr [24]. At least five field excursions are

recorded in Hole 884C, but only one is established

by a fully reverse direction. It must be pointed out, however, that the maximum field strength during

alternating field demagnetization was only 15 mT,

hence probably not sufficient to establish the charac- teristic remanent magnetization.

Finally, ODP core 769B from the Sulu Sea at 9”N

[25] (SAR = 8 cm/kyr) may be taken as one of many sedimentary records from medium and low latitudes that do not indicate a single excursion of

the geomagnetic field during the Brunhes chron.

4. Origin of contradictory field records

There are several possible causes for the

ingly contradictory evidence on the paleofield etry:

seem-

4. I. Non-dipolar field geometry during events

Geomagnetic excursions appear to be correlated with lows in the dipole field intensity [3,26,27].

Excursions can apparently be dated by the correla-

tion with lows in the relative paleointensity record [61. During polarity transitions and excursions non- dipole field terms may thus prevail. There is also the

tendency to accept any field direction near the poles as secular variation, which, of course, is only true for

the declination. A fully reversed inclination at high

latitude is only compatible with either a reversed dipole field - regardless of field strength - or with non-dipole field configurations. The latter would, as a consequence, result in very anomalous field directions in many other places around the


world. However, a large number of sedimentary field records (e.g., DSDP and ODP cores) from low- and mid-latitude sites give no indication for significant non-dipolar fields at times of events - within the limit of resolution.

4.2. Recording mechanism of sediments and temporal resolution

It is well known that sediments do not record the Earth’s magnetic field nearly as instantaneously as quickly cooled volcanic rocks. Magnetic grains tend to align themselves with the field in the water-sediment interface but the magnetization is not locked in before a certain degree of compaction prohibits phys- ical realignment with a changing magnetic field. The lock-in depth and the lock-in depth range depend on the composition of the sediment and the effect of bioturbation. Consequently, estimates for the lock-in depth vary widely and are, for example, N 1 cm [28], 16 cm [29], and > 40 cm [18]. Bioturbation, of course, is highly variable and it can be absent in anaerobic environments and in Arctic sediments. Bioturbation, and hence the lock-in depth, may thus also be climate dependent. A useful indicator for the recording fidelity of sediments is the apparent maximum directional transition duration (ATD) of a reversal, for example, the Matuyama-Brunhes (M/B) transition. Although the characterization of the whole transition should include the associated intensity os- cillations lasting more than 10 kyr [6], and possibly a directional precursor [3,19,30], the M/B decisive directional swing was apparently faster than most deep-sea sediments can resolve (due to the finite sedimentation rate) and may possibly have lasted less than 50 years [ 181. The temporal resolution by which a polarity event can be resolved is at least equal to the ATD. A decreased field intensity during the event (as evidenced) may decrease the temporal resolution, although the intensity before and during the M/B transition was also much reduced [6,19,30].

The whole length of the above mentioned ODP core 769B from the Sulu Sea was only measured with the shipboard whole-core magnetometer [25], imposing a low-pass filter; nonetheless, the M/B transition occurs over only 15 cm, amounting to an

ATD of 2 kyr. The parallel core 769A has been subsampled at 10 cm intervals for discrete specimen measurements [30] and the virtual geomagnetic pole was found to move by > 100” over 10 cm with very low relative paleointensities. Assuming that bioturbation introduced some signal smoothing, the results do not contradict even faster directional changes.

ODP cores from Site 85 1 in the equatorial Pacific Ocean have been used by Valet and Meynadier [6] for their high-resolution study of relative paleointensity. Here, the lower SAR of N 2 cm/kyr is com- pensated for by high resolution u-channel measurements. The M/B transition takes place within 2 cm and the temporal resolution for events is thus approximately 1 kyr, or > 1 kyr if the intensity is low.

Neither core 769B nor cores from site 851 show directional indications for events during the Brunhes chron. A few anomalous directions in core 769B have been correlated with slumped beds [25].

4.3, Arctic Ocean sedimentation

Average sedimentation rates in the Pleistocene were an order of magnitude higher than for the Eocene to Pliocene in the Barents Sea [31], off the continental slope of eastern Canada [32], and, presumably, in much of the Arctic Ocean. The increased sedimentation was a result of the glaciogenic envi- ronment, with decreased vegetation, increased aerial and glacial erosion, and exposed continental shelfs during low sea-level stands bringing large quantities of sediments to the deep sea [33]. During interglacials, on the other hand, with high sea levels the sedimentation may have starved [31].

The sediment cores from the Norwegian-Green- land Sea [22] recorded reverse polarities or excursions only at intervals that are barren of nannofos- sils; that is, during periods of cold climate.

If a highly variable sedimentation rate is assumed, the chronostratigraphy of the sediment cores from the Yermak Plateau of Nowaczyk et al. [l] can be reinterpreted as having high depositional rates during glaciations and low ones during interglacials. The 6 ‘*O-based age determinations for core PS 1533-3 allow such an interpretation. The 14C ages indicate increased sedimentation during the last glacial. The

H.-U. Worn/Earth and Planetary Science Letters 147 (1997) 55-67 59

high proportion of reverse polarity intervals in these

cores suggests a link between polarity events and

glaciations.

5. Evidence from volcanic rocks

Owing to the episodic nature of volcanism a short excursion of the geomagnetic field may be easily

missed even in a nearly continuous sequence of lava

flows. However, given a large enough data base some statistical evidence on the overall duration of

reverse polarity intervals during the Brunhes chron can be gained. Mankinen and Dalrymple [34] listed

about 50 dated volcanic rocks younger than 730 ka, none of which have reverse polarity. Recently, Holt

et al. [35] reported on the field inclination of 195 lava flows, with a maximum age of 400 ka, recov-

ered during the Hawaii Scientific Drilling Project.

Two of the flows are of reverse polarity. A broader data base is not readily available and statistics on all

dated and paleomagnetically measured volcanic rocks

may be biased because excursions and reversals have been particularly targeted for paleomagnetic studies.

Nonetheless, 1% appears to be the order of magnitude for the total duration of polarity events during

the Brunhes chron. If 10 or 12 events occurred during the Brunhes chron [ 1,3,6] the average dura-

tion of an event would be around 0.5 kyr.

6. Evidence from loess / paleosol deposits

Loess is an aeolian continental sediment that ac-

cumulated across former periglacial regions during the ice ages. The warmer and more humid climate

during interglacials provoked alteration of loess into

soils (paleosols). Several loess/paleosol sequences have been dated by magnetostratigraphic studies [4]. The first successful dating of the Chinese Loess

Plateau was performed by Heller and Liu [36] by determining the inclinations of 231 borehole samples spanning a profile of about 150 m. Loess deposition

commenced immediately after the Gauss/Matuyama (G/M) polarity reversal, which was recorded in an underlying clay layer. The Olduvai and Jaramillo subchrons, as well as the Matuyama/Brunhes boundary (MBB), were identified. All polarity boundaries except for the older Olduvai and the

MBB were recorded in the loess. However, after

further sampling, Heller and Liu [37] also assigned the older Olduvai boundary to a loess section. A

nearly simultaneous study by Torii et al. [38] only a

few kilometres away also found the MBB in loess

deposits. At least four subsequent loess magne-

tostratigraphy studies [39-421 found the MBB in Ioess rather than in paleosols. Results obtained at

Baoji [40], a few hundred kilometres away, yielded

essentially the same results as from Luochan, except for that the G/M boundary was found in loess while

the Jaramillo boundaries were not well defined, pos-

sibly as a result of incomplete laboratory “cleaning”

[41. Despite high sedimentation rates, geomagnetic ex-

cursions have rarely been identified in loess. Cham- pion et al. [20] cite a number East European studies

with presumed records of events in loess; however,

these are not sufficiently documented. The Blake event has been identified as two or

possibly three successive full field reversals in Chi-

nese [43,44] and German [45] loess sections. They agree in their stratigraphic position immediately

above the paleosol of the Eemian interglacial ( _ 120

kal. The discussed duration of the event is N 5 kyr for the Chinese loess [43&l, while evidence pre-

sented for the German sections indicates a much

shorter duration [45]. Thermoluminescence age determinations for the top and bottom of the 1 m loess

interval resulted in indistinguishable ages of 110 + 8

ka and 115 k 9 ka, but the whole interval was cer- tainly deposited within 10 kyr [45]. The paleomag-

netic results of the very densely sampled section

show that the individual reversals extend over only 5

cm or less, indicating that an entire reverse field interval may have lasted less than 500 years.

Recently, the Mono Lake event has also been identified in other loess deposits in Germany as fully reverse directions of short duration [46]. The short duration of excursions and the mostly rather wide

sampling density may explain why other excursions have not yet been identified in loess sections.

Despite a few conflicting results, apparently related to the problem of extracting the primary remanence from weakly magnetized and multicomponent loess and paleosol samples, the existing paleomagnetic results obtained on loess sequences indicate that possibly all geomagnetic field reversals since the

60 H.-U. Worm/ Earth and Planetary Science Letters I47 (1997) 55-67

0.4 0.5 0.6 0.7 0.8

Age WI

Fig. I. Relative paleointensity record obtained on sediments from the equatorial Pacific by VaIet and Meynadier [6] (lower curve) and the

oxygen isotope curve of Shackleton et al. [5] (upper curve). Warmer climate down, cooler climate up. The (Pearson) correlation coefficient

is r = 0.50.

Gauss/Matuyama boundary occurred during periods of loess deposition and thus presumably during glaciations.

7. Relative paleointensity records and climate

Because the ages of several events are poorly constrained it was intended to refine these ages by tying them to lows in relative paleointensity records of well dated sediments, as suggested by Valet and

Meynadier (V&M) [6]. However, the comparisons of the oxygen isotope record with the relative paleointensity records of V&M [6] and Tauxe and Shackleton (T&S) [7], respectively, show startling correspondences (Figs. 1 and 2) that have not been discussed by the authors. The correlation coefficient for the S I80 curve [5] and the V&M record up to 0.78 Ma is r = 0.50, while it is only r = -0.045 for the T&S record and the a’*0 curve. Still, the correspondence of certain features in both curves is quite obvious. The low correlation coefficient is a

1~“‘l”“l”“l”“rlllllllllllllllllll~””~ 0.0 0.1 0.2 0.3 0.4

Age [MaI

Fig. 2. Relative paleointensity record obtained on sediments from the Ontong-Java Plateau by Tauxe and Shackleton [7] (upper curve) and

the oxygen isotope curve of Shackleton et al. [5] (lower curve). Warmer climate up, cooler climate down. The (Pearson) correlation

coefficient is r = - 0.045.

H.-U. Worm/Earth and Planetary Science Letters 147 (1997) 55-67 61

result of phase shifts to sometimes younger, sometimes older ages and of different amplitudes. At first, this seems to suggest that paleointensity is also modulated by climate (or vice versa); however, it is decisive to note that, for the V&M curve, intensity maxima correspond to temperature minima while the opposite is true for the T&S record (the S ‘*O curve is once plotted in ascending and once in descending order). Both intensity records are incompatible, their correlation coefficient is r = -0.017. Since we are dealing with a mainly dipolar field the intensity must have varied similarly at the different geographic locations. Therefore, it seems likely that the applied normalization of the NRM intensity by the rock magnetic parameters susceptibility, ARM or IRM, was insufficient in removing climatically driven modulations of the rock magnetic parameters.

more recent (and therefore more precise) radiometric datings are around 42 ka, which coincides with minima of both paleointensity records (Figs. 1 and 2).

8.3. Norwegian-Greenland Sea and Fram Strait

It is not clear whether these were separate or identical events. The discussed age brackets of 65-82 ka for the Norwegian-Greenland Sea event can be reconciled with a minimum in the V&M record at 75 ka, which, however, is a suspect rock magnetic value. There is no minimum in the T&S record for these age brackets. The next older paleointensity minimum of the V&M curve at 9 1 ka may possibly be correlated with two excursions found in Lac du Bouchet [2] and in the Fram Strait [l]. The T&S record has a minimum at about 100 ka.

8. Ages of geomagnetic reversals and events 8.4. Blake

Chron and subchron boundaries have now pre- cisely been dated for the last few million years [lo, 1 I], but age determinations for some geomagnetic events and estimates for their durations scatter widely (see [ 1,311. For brevity not all age determinations for the presumably 17 events since the Gauss/Matuyama boundary can be discussed and reviews are referred to in [ 1,3,20]. In addition, minima in the V&M and T&S intensity records are used for refined ages, where the minima are not suspected to be a climatically driven rock magnetic signals.

The Blake event is documented by fully reverse field directions for at least six well separated sites [I ,35,43-45,471. It has also been well dated at 117 ka by oxygen-isotope stratigraphy in cores from the Mediterranean Sea [47], in agreement with the loess studies [43-451, where it is documented in loess deposits immediately overlying the Eemian interglacial paleosols.

8.5. Biwa I, II and III

8.1. Mono Luke

Nowaczyk et al. [I] listed 13 references for the Mono Lake event and the assigned ages average 24 ka with little scatter. This event corresponds to a minimum in the T&S record [7] (Fig. 1). However, it is also close to the minimum of the last glacial maximum in the 6 ‘* 0 curve.

Three large excursions older than the Blake event were recorded in Lake Biwa, Japan. All four events (Blake, Biwa I-III) occur at depths where the or- ganic carbon content of the sediments is at a minima [48], thus indicating cold climatic conditions.

8.2. Laschamp

The earliest identified excursion (in 1966) is documented by 26 publications and has assigned ages that scatter from 32 to 50 ka [ 1,3]. However, the

Ages from 160 to > 200 ka are discussed for Biwa I in seven publications [l]. This event may be identical to the one termed Baffm Bay, which was also recorded in loess in Alaska, 3 m below a tephra dated at 149 f 13 ka (see [20]). The V 8zM record has sharp minima at 187 and 195 ka (Fig. l), the T&S record at 192 ka (Fig. 2). The two younger ages are in agreement with an occurrence during a cold climate.

The ages discussed for Biwa II in nine publications scatter widely, from 252 to 344 ka [l]. The

62 H.-U. Worm/ Earth and Planetary Science Letters 147 (I997) 55-67

T&S record has a minimum at 272 ka, and the V&M record at 282 ka but a rock magnetic origin appears possible (Fig. 1).

Age estimates for Biwa III range from 350 to 400 ka [1,20]. The V&M record has minima at 368, 402 and 413 ka, while the T&S record has a single minimum at 386 ka. An age of 413 ka corresponds to a warm climate and is presumably too old.

8.6. Emperor

Age estimates range from 417 to 490 ka [ 1,201. Based on the occurrence in Elster II glacial deposits, Champion et al. [20] assigned an age of 470 ka. This is in agreement with results from the Norwegian- Greenland Sea, where a reversal was found in the cold stage 12 [22]. The V&M record has a minimum at 469 ka.

8.7. Big Lost

Basalts from Idaho that recorded an excursion have been dated at 565 f 14 ka [20]. Recently, Champion et al. [49] revised the age to 550 f 10 ka. The V&M record has a minimum at 554 ka. This event may also correspond to the excursion found in basalts from West Eifel, Germany, dated at 510 f 30 ka [27].

8.8. Delta

This event has been detected in deep-sea sediments, in sediments from Italy (see [2011, and in a Chinese loess section [41]. The age assignments of approximately 620 ka for the previous two sites are related to an assumed age of 730 ka for the Brun- hes-Matuyama boundary, so that the revised age is more likely 660 ka. The susceptibility record of the Chinese loess can possibly be linked to the oxygen isotope curve, where the event is situated on stage boundary 16/17, in agreement with an age of 660 ka. The V&M record has a very sharp minimum at 660 ka.

8.9. Matuyama / Brunhes boundary

The M/B boundary (MBB) is the best character- ized polarity boundary and, because of its strati-

graphic position close to the crest of isotopic stage 19.1, it simultaneously presents some contradictory evidence that deserves discussion. The age of the MBB is narrowly constrained to 778.7 f 1.9 ka by several 4”Ar/ 39Ar age determinations [SO], while the oxygen-isotope curve is independently tuned to astronomical parameters (i.e., the insolation variation at 65”N in July). The crest of isotopic stage 19.1 has been determined to an age of 784 ka [5]. Tuning to an insolation curve for June or August would result

Table 1

Ages of geomagnetic events and polarity boundaries

Name Age References

(ka)

Mono Lake

Laschamp

Fram Strait

Blake

Biwa I

Biwa II

Biwa III

Emperor

Big Lost

Delta

Matuyama/Brunhes

Kamikatsura

Younger Jaramillo

Older Jaramillo

Cobb Mountain

Ontong Java I

Ontong Java II

Gilsa

Younger Olduvai

Older Olduvai

Reunion

Gauss/Matuyama

24 A

42 A-B 91 B

100 A

117 C

187 B

192 A

272 A

386 A

368 or 402 B

469 B

550* 10 D

554 B

660 B

778.7* 1.9 E

780 F,G 931 or 945 B

987 B

990 F,G 1060 F

1070 B,F,G 1193 B

1201-1211 G

1370 B

1440 B

1675 B

1770 G

1780 F

1785 B

1950 B,F,G 2135 B

2140-2150 F.G 2581 G

2600 B,F

A = Minimum in the Tauxe and Shackleton intensity record [7].

B = Minimum in the Valet and Meynadier intensity record [6]. C = Tucholka et al. 1471. D = Champion et al. 1491. E = Singer

and Pringle [50]. F = Baksi [lo]. G = Cande and Kent 1111.

H.-U. Worm/ Earth and Planetary Science Letters 147 (1997) 55-67 63

in a shift of 2 kyr for the oxygen-isotope curve [Sl]. Supposing that both dating uncertainties amount to not more than 4 kyr it is evident that the M/B transition occurred in a cooling climate. This is in agreement with the MBB being found in loess rather than in paleosols.

It is interesting and important to note that some

astronomically tuned ages obtained by determining the relative stratigraphic position of the MBB in marine sediments with respect to the oxygen-isotope curve yielded older ages. Johnson [12], in his semi- nal but long not accepted paper, assigned an age of 788 + 5 ka because, in the cores under study, the MBB was found below stage 19.1, but he noted that

(4

03 0.4 0.5 0.6 0.7 0.8 0.9

fw WI

(W

0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.6

Age Hal Fig. 3. The oxygen isotope record 6’*0 of 151 (a) O-9 Ma; (b) 9-2.6 Ma, thought to represent global ice volume and ages of geomagnetic

events and polarity reversals (Table 1). The temperature increases with larger (negative) numbers. The width of shaded areas does not

represent the duration of events or reversals but reflects the uncertainty in age. The ages of events and reversals correspond to times of

global cooling.


the relative positions depend on sedimentation rate and he held the post-depositional remanence acquisition process responsible.

It took 8 more years before an astronomically tuned age of 780 ka for the MBB [5] became accepted and the long held K-Ar age of 730 ka was abandoned. Ironically, the timescale of Shackleton et al. [5] is based on cores from ODP Site 677, for which a magnetostratigraphy has never been determined (!I. The positions of the reversal boundaries were estimated. Moreover, the authors placed the MBB at approximately the same position as Johnson [12], below stage 19.1 and thus at an older age than 784 ka.

Berger et al. [51] also found the MBB between stages 19 and 20 in sediment cores from the On- tong-Java Plateau. They arrive at an age of 790 f 6 ka and acknowledge that the difference with the radiometric age may result from the subsurface recording of the magnetic signal. The reason for a stratigraphic position of the MBB below stage 19.1 in many marine sediments (see [28,29]) is most likely the subsurface recording process in sediments, with a lock-in depth of the order of IO-20 cm, as deduced by deMenoca1 et al. [29], although Tauxe et al. [28] argue against a significant lock-in depth. But even from their compilation (fig. 3 of 1281) it is evident that the stratigraphic position of the MBB

relative to isotope peak 19.1 clearly moves downward to older ages with decreasing sedimentation rates. The acceptance of post-depositional remanence acquisition in deep-sea sediments also avoids the discussion on large phase lags (- 10 kyr) between the continental (loess) and marine climatic records, correlation errors between loess/palesol sequences and the oxygen-isotope record [28], huge lock-in depths (> 1 m> in loess, and suggested time lags of several thousand years for the MBB between the Indian-Pacific and Atlantic regions [28].

Ages of the older polarity boundaries and six more events are adopted from the recent GPTS [ 10,111 and the paper of Valet and Meynadier [6] (Table 1).

9. The ages of events and reversals compared with the oxygen-isotope record and orbital parameters

The oxygen-isotope record of Shackleton et al. [51 is thought to represent changes in global ice volume and sea level variations. Fig. 3 shows the S “0 record together with the discussed ages of the events and polarity boundaries (Table 1). It is clear that all ten events during the Brunhes chron may have occurred at times of global cooling (Le., during glaciations).

0.4

Age [MaI

Fig. 4. Ages of events during the last 800 kyr and variations in the astronomical parameters precession, obliquity, and eccentricity of the

Earth’s orbit.

H.-U. Worm/Earth and Planetar? Science L_.etters 147 (I 9971 .%67 65

The situation for the older events and reversals is not quite as obvious (Fig. 2b): the climatic variations were less severe and some reversals and events occurred at times of small amplitude cooling (Younger Jaramillo and Gilsa); others are located at times of glacial(?) maxima rather than on the cooling shoulder of the isotope curve (Cobb Mountain, On- tong-Java I, Younger and Older Olduvai and Re- union). Also, the age spans given for the Gauss/Matuyama (2.58 l-2.60 Ma) and the Older Jaramillo (1.06-1.07 Ma) cover a warming and a cooling period. However, the detection of all polarity boundaries since the Gauss/Matuyama transition in loess rather than in paleosols supports the link of reversals to glaciations for the older chron boundaries as well.

Since the Earth’s climate is driven by astronomical forces a common cause for the reversing geodynamo could be suspected. However, there is no convincing correlation between events and reversal ages and the orbital parameters eccentricity, obliquity, and precession (Fig. 4).

10. Conclusions

All 16 geomagnetic events, whether excursions or short polarity reversals, and all six polarity boundaries since the Gauss/Matuyama boundary appear to be linked to glaciations, although some age determinations are still too vague for a definite correlation. Relationships between reversals/excursions and climate have been suggested before (see Jacobs [3] for a discussion) but mostly turned out to be untenable. For example, the contention of a 100 kyr cyclicity of excursions [52], and thus a link to the eccentricity of the Earth’s orbit, is disproved by their age distribu- tion. There is also no correlation with the parameters precession or obliquity (Fig. 4). Astronomical forces can thus be ruled out as triggers for reversals. The Earth’s climate is tuned to the insolation curve at 65”N, which is not a force acting on the Earth’s core. Therefore, the mechanism causing events and reversals is presumably the altered momentum of inertia of the Earth, due to a reduction in the sea-level, leading to an increased rotational velocity (- 1 s/ day). The position of most events on the cooling flanks rather than minima of the oxygen-isotope curve suggests that the geodynamo is affected by the

acceleration of the Earth’s rotation rather than by the increased velocity itself. A correlation between changes in the day length and the geomagnetic field has been noted for historic times [53]. It is not suggested, however, that all previous polarity reversals were also linked to climatic changes.

A discussion of the stratigraphic position of the MBB demonstrates that the comparison of events and reversals with the oxygen isotope record is not straightforward. The magnetic signal tends to be displaced downward. Unfortunately, there is no generally valid phase lag. The lock-in depth and the lock-in depth range depends on the composition of the sediments, probably the sedimentation rate, and the effect of bioturbation. The latter may, in turn, be strongly dependent on climate. The value of 16 cm deduced by deMenoca1 et al. [29] can only be an order of magnitude estimate.

Most events were apparently very short-lived, < 1 kyr on average, and may consist of multiple “spikes”, as documented for the Blake event [43- 451. The Blake event, at least, constituted a fully reversed dipolar field because it is documented by reverse field directions for at least six well separated sites [1,3,35,43-451. However, it has to be empha- sised that a fully reversed field direction documented for only one site is compatible only with a reversed dipolar field, or with higher order multipole fields, which should then be recorded as excursions world- wide.

The spike-like events are mostly obscured in many sediment cores because of limited resolution, owing to relatively low sedimentation rates; signal smoothing, due to a lock-in depth range; bioturbation; and viscous overprints of the presumably low-amplitude signals. The apparent duration of geomagnetic events is exaggerated in sediments at high latitudes because of increased sedimentation during glaciations and the concurrent occurrence of events.

The directional transition at the Matu- yama/Brunhes boundary may have taken place within 100 years. The paradigm that the Earth’s magnetic field requires a few thousand years for a reversal should thus be abandoned. If field reversals were generally fast, then a large number of studies on reversal paths (see [3]) have revealed nothing more than the signal-smoothing process by the recording mechanism in sediments.

66 H.-V. Worm/Earth and Planetary Science Letters 147 (1997) 55-67

A correlation between paleointensity and climate may also be suspected. However, the apparent correspondences between the relative paleointensity records of Valet and Meynadier [6] and Tauxe and Shackleton [7] and the oxygen-isotope curve cannot both be true because paleointensity correlates with temperature in [7] while it anti-correlates in [6]. Certain features of other relative paleointensity records also resemble climatic variations, particularly the increase in intensity since the last glacial maximum (see [54]). The present criteria for obtaining relative paleointensities from sedimentary records may be insufficient and should be more stringently refined.

Acknowledgements

Financial support from the Deutsche Forschungs- gemeinschaft (DFG) is gratefully acknowledged. fZtV3

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Documents

A link between geomagnetic reversals and events and glaciations