Tiedemann, R., Mix, A.C., Richter, C., and Ruddiman, W.F. (Eds.) Proceedings of the Ocean Drilling Program, Scientific Results Volume 202
2. LATE QUATERNARY PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY FROM ODP SITES 1233 AND 12341
Steve P. Lund,2 Joseph Stoner,3 and Frank Lamy4
ABSTRACT
During Ocean Drilling Program Leg 202, detailed shipboard paleo- magnetic measurements at Sites 1233 and 1234 identified a reproduc- ible high-resolution record of paleomagnetic secular variation (PSV) at each site. These records also contained evidence for three magnetic 1Lund, S.P., Stoner, J., and Lamy, F., field excursions. This paper summarizes the complete paleomagnetic 2006. Late Quarternary paleomagnetic record from Site 1233 and the paleomagnetic record for the uppermost secular variation and chronostratigraphy from ODP Sites 30 meters from Site 1234. We also establish a detailed PSV correlation 1233 and 1234. In Tiedemann, R., between the two sites, located ~500 km apart. Twenty-seven accelerator Mix, A.C., Richter, C., and Ruddiman, mass spectrometry radiocarbon dates permit us to develop time-depth W.F. (Eds.), Proc. ODP, Sci. Results, 202: plots and detailed PSV time series for the last ~50,000 yr at both sites. College Station, TX (Ocean Drilling Based on our chronostratigraphy, the most recent excursions occur at Program), 1–22. doi:10.2973/ α odp.proc.sr.202.208.2006 ~35,000 cal. yr before present (BP) (Excursion 3 ) and ~41,000 cal. yr BP 2Department of Earth Sciences, (Excursion 3β). The excursions were named for the oxygen isotope University of Southern California, Los stages within which they occur. These two excursions are not signifi- Angeles CA 90089-0740, USA. cantly different in age from the Mono Lake Excursion and Laschamp [email protected] 3 Excursion. A third, older excursion, which has not yet been adequately School of Oceanic and Atmospheric Sciences, Oregon State University, dated, also occurs at Site 1233. Corvallis OR 97331-5503, USA. 4GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, INTRODUCTION Germany.
Our knowledge of detailed geomagnetic field secular variation in the Initial receipt: 4 November 2004 Acceptance: 23 February 2006 Southern Hemisphere is very limited during prehistoric times. A few Web publication: 1 December 2006 Ms 202SR-208 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 2 leomagnetic studies have developed reproducible Holocene paleomag- netic secular variation (PSV) records from Australia (Barton and McElhinny, 1981; Constable and McElhinny, 1985), New Zealand (Turner and Lillis, 1994), Argentina (Creer et al., 1983; Gogorza et al., 2000, 2002), and Antarctica (Brachfeld et al., 2000), but there is no re- producible detailed record of PSV from the entire Southern Hemisphere prior to ~18 ka. A gridwork of such records is essential for better under- standing the global space-time pattern of field variability between re- versals. Such a detailed knowledge of space-time field variability is particularly necessary to properly test newer, more realistic dynamo models of field variability (e.g., Kono and Roberts, 2002; Wicht and Ol- son, 2004) and better evaluate the relationship between magnetic field excursions and normal secular variation (e.g., Lund et al., 2001). During Leg 202, we recovered replicate high-resolution PSV records for the last ~70–150 ka from Sites 1233, 1234, and 1235 (Mix, Tiede- mann, Blum, et al., 2003). We present here a summary of the shipboard PSV data from replicate holes at Sites 1233 and 1234 for the last 50,000 yr. The PSV records from both sites contain evidence for two magnetic field excursions that are coeval with the Mono Lake (Benson et al., 2003) and Laschamp (Lund et al., in press) excursions, observed in the Northern Hemisphere. We also correlate the PSV records from Sites 1233 and 1234 and note their strong similarity even though they are lo- cated ~500 km apart. Finally, we integrate 27 radiocarbon dates recov- ered from Site 1233 to develop a revised chronostratigraphy at Sites 1233 and 1234 for the last 50,000 yr.
PALEOMAGNETIC METHODS
Shipboard paleomagnetic measurements were made on all cores re- covered during Leg 202. Individual cores, normally ~9.5 m in length, were cut into 1.5-m sections and split in half; each archive half was measured at 5-cm intervals on the shipboard 2-G Enterprises cryogenic magnetometer (Mix, Tiedemann, Blum, et al., 2003). The natural rema- nent magnetization (NRM) of each core segment was initially measured and then alternating-field (AF) demagnetization at one or two steps up to 25 mT was applied; the NRM was remeasured after each AF demagne- tization step. Individual core segments were not demagnetized at higher AF because the archive halves were to be further sampled for more detailed paleomagnetic studies after the completion of Leg 202. The limited shipboard AF demagnetization up to 25 mT, however, did a good job of removing a strong but soft viscous overprint that is often noted in cores collected during Ocean Drilling Program (ODP) legs (Mix, Tiedemann, Blum, et al., 2003; Lund et al., 2003). More detailed paleomagnetic measurements have been made in three selected 1.5-m intervals using U-channels that were collected from the centers of ar- chive core halves. These results, reported in a companion article (Lund et al., this volume), corroborate the shipboard paleomagnetic measure- ments presented here. All holes at each site have been correlated based on a variety of ship- board stratigraphic measurements and normalized to a common depth scale (meters composite depth [mcd]) for comparison (Mix, Tiedemann, Blum, et al., 2003). A composite splice section has been chosen from the replicate holes that best represents the observed variability at each site. These independent correlations were used to reconstruct the decli- nation record at each site. The declination records have been modified S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 3 by first rotating the average declination for each core to 0° and then F1. Site 1233 PSV records, 0–30 slightly varying each core’s average declination to best match the decli- mcd, p. 11.
A Composite PSV splice Hole 1233A Hole 1233B Hole 1233C Hole 1233D Hole 1233E nation trends between adjacent cores from each hole based on the ob- -80° -60° -60° -60° -60° -60° -60° -40° -20° 0 I1 260 I2 I3 1,180 I4 1,980 I5 3,820 5 I6 I7 5,040 I8 served declination variability in the other holes using the splice I9 5,590
I10 7,090 I11 10 8,780 I12 I13 I14 10,040 correlations. I16 I15 15 I18 I17 I19 12,260 I20 I21 20 I22
Sediment depth (mcd) 13,620 I23 I23 ‘ I24 I25 25 I26 17,240 I27 19,680 I28 Cal. yr BP 30 RESULTS B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20°0° 20° 0° 20° 0° 20° 0° 20° 0° 20° 0 260 D2 D1 D3 1,180 D4 1,980 D5 3,820 5 D6 D7 5,040 D8 5,590 D9 D10 7,090 10 8,780 Site 1233 Paleomagnetic Secular Variation Records D11 10,040
15 D12 D13 D14 12,260 D15 D16 20 Sediment depth (mcd) 13,620 D17 D17 ‘ D18 The AF demagnetized paleomagnetic directional records from Holes D19 25 D20 17,240 D21 D22 19,680 D23 Cal. yr BP D24 1233A–1233E (41.0°S, 74.45°W; 838 m water depth) are displayed in 30 D25 Figures F1, F2, F3, F4, and F5. The solid lines represent a seven-point running average of the directional data. Selected inclination (I1–I106) F2. Site 1233 PSV records, 30–60 and declination features (D1–D111) are noted based on their appear- mcd, p. 12. ance in records from multiple holes. Normally, directional features were A Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -80°-60° -60° -60° -60° -60° -40° 30 I29 I30 ( I31 ) 21,480 replicated in at least three holes, but selected features (in parentheses) ( I32 ) 35 I33 22,510 I34
I35 40 23,860 were only replicated in two holes. The boundaries between successive I36 I37 I38
26,050 I39 45 cores are evident in Figures F1, F2, F3, F4, and F5 as gaps in the direc- I39 ‘ 28,830 I40 29,510 I41
Sediment depth (mcd) 50 ( I42 ) 29,780 ( I43 ) tional records. The paleomagnetic directional record from the splice is ( I44 ) ( I44 ‘) 55 I45 I46 32,340 I47 I48 I49 36,360 I50 Cal. yr BP shown at the left in Figures F1, F2, F3, F4, and F5. The splice directional 60 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20° 0° 20° 0°20° 0° 20° 0° 20° 0° 20° 40° 60° 80° 30 record represents our best composite record of the PSV recorded at Site D26 D27 21,480
35 D28 D29 22,510
1233, based on shipboard measurements. D30 40 D31 23,860 D32
D33 ° ° 26,050 B B D34 The inclinations at Site 1233 typically vary from 70 to 30 with an 45 D35 D35 ‘ 28,830 D36 29,510 D37 ° ° Sediment depth (mcd) 50 B D38 29,780 average near 55 . The observed average inclination is ~5 less than the Interval of D39 more uncertain D40 correlations 55 D41 32,340 D41 ‘ ° D42 expected axial-dipole inclination for the site latitude and ~10 less than D43 Excursion 3α 36,360 D44 Cal. yr BP 60 the average inclination expected for this site based on long-term lati- tude-dependent nondipole effects (ΔI anomaly) (e.g., Lund, 1993; Mer- rill et al., 1996). The slightly lower than expected inclinations are F3. Site 1233 PSV records, 60–90 probably a result of the presence of some residual viscous overprint mcd, p. 13. A Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -80° -60°-60° -60° -60° -60° -40° (steep positive inclination) from coring as noted above (Mix, Tiede- 60 I51
I52
65 I53 mann, Blum, et al., 2003; Lund et al., 2003). The limited U-channel (I54) I55 Excursion 3β
70 (I56) I57 ° ° I58 I59 measurements presented in Lund et al. (this volume) document 5 –10 I60 75 I61 I62 I63 I64 I65 I66 I67 steeper inclinations in the cleaned U-channel paleomagnetic results but Sediment depth (mcd) 80 (I68) (I68) (I69) (I70) I71 I72 I73 85 I74 show the same pattern of variability noted in the shipboard measure- I75 I76 I78 I77 90 ments presented here. The inclination variability displays a strong sense B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20°0° 20° 0°20° 0° 20° 0° 20° 0° 20° 60 D45 D46 of cyclicity overall. The most common cyclicity occurs with a strati- D47 D47 ‘ 65 D48 D49
D50 Excursion 3β
70 D51 graphic interval of ~1–2 m, but longer ~3- to 5-m cycles are also inter- D52 D53 D54 D55 D56 75 D57 D58 mittently visible. D59 D59 ‘ D60
Sediment depth (mcd) 80 D62 D61 D63 (D63?) ° ° (D64?) The declinations typically vary from B20 to +20 with an artificially D65 Interval of 85 D66 more uncertain D67 correlations D68 D69 ° D70 imposed long-term average declination near 0 . The declination vari- 90 D71 ability is higher between ~45 and 70 mcd, with 60°–70° declination swings being more common. This interval contains two magnetic field excursions, and the declinations display the same overall pattern of cy- F4. Site 1233 PSV records, 90–120 clicity noted for the inclinations above. mcd, p. 14. A Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -60°-30° -60° -60° -60° -60° -30°0° 30° 90 I80 I79' Radiocarbon age determinations and an estimated chronology for (I81) (I82) Zone of 95 questionable but reproducible Site 1233 based on preliminary paleointensity measurements (Mix, ?I83? directions I84 100 I85 I86
Intervals of Tiedemann, Blum, et al., 2003) suggest that Site 1233 has an average 105 magnetic mineral dissolution I87 I88 I89 110 I90 Sediment depth (mcd) (I91) Unnamed excursion sediment accumulation rate of ~1.5 m/k.y. Based on this rate, the incli- (I92) Zone of 115 unreproducible inclinations
(I94) nation and declination cyclicity duration ranges primarily from ~700 to 120 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20°0° 20° 0°20° 0° 20° 0° 20° 0° 20° 40° 60° 90 3500 yr. Such values are quite similar in amplitude and frequency to D72 (D73) (D74) 95 (D75) D76 late Quaternary PSV observed in the Northern Hemisphere (e.g., Lund, D77 D78 D79 D80 100 D81 D82 D83 Intervals of (D84) magnetic mineral D85 1993, 1996). 105 dissolution D85 ‘ D86 D87 D87 ‘ D88 110 D89 Sediment depth (mcd) D90 Unnamed excursion D91 The inclinations and declinations also display six short intervals of D92 (D93) (D94) 115 (D95) (D96) (D97) significantly higher variability: (1) 57–59, (2) 66.5–69, (3) 95.5–99.5, (4) (D98) 120 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 4
109–110.5, (5) 112.5–117.5, and (6) 120–125 mcd. We believe that the F5. Site 1233 PSV records, 120–140 directions in three of these intervals (95.5–99.5, 112.5–117.5, and 120– mcd, p. 15.
A Composite PSV splice Hole 1233D Hole 1233C Hole 1233C Hole 1233B 125 mcd) are anomalous because of coring or rock magnetic complica- -60°-30° -60° -60° -30° 0°30° 0° 30° 120 (I95) (I96) tions. The intervals at 95.5–99.5 and 120–125 mcd contain inclinations (I97) 125 ???
(I98) (I99) that reach almost reversed polarity and are reproducible in three holes. (I100) (I101) 130 (I102) (I103) (I104)
However, the declinations in both intervals are not at all anomalous, Sediment depth (mcd) (I105) (I106) and rock magnetic evidence (Mix, Tiedemann, Blum, et al., 2003) indi- 135 140 B Composite cates that these are intervals of extremely low NRM intensities. The PSV splice Hole 1233B Hole 1233C Hole 1233D -20°0° 0° 0° 0° 20° 120 (D99) (D100) third interval at 112.5–117.5 mcd has no reproducible inclination mea- (D101) (D102) 125 ??? (D103) (D104) (D105) surements, even though the declinations appear to be normal, and also (D106) (D107) 130 (D108)
(D109) occurs in an interval of extremely low NRM intensities. More detailed (D110) Sediment depth (mcd) (D111) U-channel paleomagnetic and rock magnetic measurements will be nec- 135 essary to better understand the observed directional variability in these 140 three intervals, but we now consider the directional variability to be an artifact of coring or rock magnetic complications rather than that of geomagnetic origin. We believe that the directions in the other three intervals are of geo- F6. Excursion 3β detail, Site 1233, magnetic origin and represent magnetic field excursions. The declina- p. 16. tions in the interval 57–59 mcd reproducibly undergo an ~90° swing to A Composite PSV splice Hole 1233C Hole 1233D -60° -20° -60° -20° -60° -20° the east and a return to values near 0°, whereas the inclinations vary in 64 65 (I53) a more normal manner. It is important to note that this anomalous dec- 66 (I54) 67
(I55) lination swing is seen in all holes, which were separated by a total dis- 68
Sediment depth (mcd) 69
(I56) tance of at least 100 m. Our initial assessment was that the anomalous 70 71 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D -50° 0° -50° 0° -50° -0° -50° 0° declinations were a sedimentologic artifact, perhaps caused by a local 64 D47'
65 (D48) (D49) sediment slump or turbidite. However, this exact directional pattern is 66 also seen at Site 1234, ~500 km away. The directions are truly excur- 67 68 D50
° Sediment depth (mcd) 69 sional (virtual geomagnetic poles [VGPs] >45 from the North Geo- (D51) 70 D52 graphic Pole) in the interval 57.5–58.5 mcd. The directions in the 71 interval 66.5–69 mcd (Fig. F6) reproducibly undergo a complete mag- netic field reversal that contains truly excursional directions within the F7. Site 1234 PSV records, 0–15 entire interval. The onset of the excursion and almost complete reversal mcd, p. 17.
A Composite of polarity occurs within <50 cm, and directions maintain stable re- PSV splice Hole 1234A Hole 1234B Hole 1234C -60°-40° -40° -40° -40° -20° 0 (I1) (I2) I3 I3'' versed polarity for more than a meter, finally returning to normal polar- I4 I5 I6 I7 I8 Interval of anomalously low inclinations 5 ity with some significant oscillation in another meter. A similar full I10 I9 I10' I12 I11 I13 I14 I15 I16 I16 ‘ I17 reversal occurs in the interval 109–110.5 mcd. All three excursion inter- I17 ‘ I18 10 I18' Sediment depth (mcd) I19 I20 I21 I22 vals occur within intervals of surrounding PSV that are reproducible I22 ‘ I23 I24 I25 I26 15 among all holes at Site 1233. B Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -20°0° 20°0° 20° 0° 20° 0° 20° 0 D2 D1 D3 D3' D3'' D4 D5
D6 D7 5 D8 D9 Site 1234 Paleomagnetic Secular Variation Records D10 D10 ‘ D12 D11 D13 D14 D13 ‘ D15 D16
10 D18 D17
Sediment depth (mcd) D18 ‘ D19 D20 The AF-demagnetized paleomagnetic directional records for the up- D21 D22 D24 D23 D25 permost 30 mcd in Holes 1234A–1234C (36.22°S, 73.68°W; 1015 m wa- 15 D26 ter depth) are shown in Figures F7 and F8. The solid lines represent a three-point running average of the directional data. Selected inclina- F8. Site 1234 PSV records, 15–30 tion (I1–I45) and declination features (D1–D47) are noted based on mcd, p. 18. their appearance in records from at least two holes. The boundaries be- A Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -60°-40° -40° -40° -40° -20° 0° 15 I27 I28 tween successive cores are evident in Figures F7 and F8 as gaps in the di- I29 I30
I31 I32 rectional records. The paleomagnetic directional record from the splice I33 20 I34
I35 I36 is shown at the left in Figures F7 and F8. The splice directional record I37 Excursion 3β
I38
Sediment depth (mcd) 25 I39 I40 I41 represents our best composite record of the PSV recorded at Site 1234, I42 I43
I44 I45 based on shipboard measurements. Analysis of the lower portions of 30 B Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -30°0° 0° 0° 0° 30° 15 this site is continuing. D28 D27 D29 D30 D31 D32 ° ° D33 D34 The inclinations at Site 1234 typically vary from B50 to B10 with an Excursion 3α D35 20 D36 D37 ° ° D37' average near B40 . The observed average inclination is ~15 less than D38 Excursion 3β D39 D40 D41 25 D42 Sediment depth (mcd) the expected axial-dipole inclination for the site latitude and ~20° less D43 D44
D45
D46 than the average inclination expected for this site based on long-term 30 D47 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 5 latitude-dependent nondipole effects (ΔI anomaly) (e.g., Lund, 1993; Merrill et al., 1996). The lower than expected inclinations are probably a result of the presence of some residual viscous overprint (steep posi- tive inclination) from coring (Mix, Tiedemann, Blum, et al., 2003; Lund et al., 2003). Limited U-channel measurements from Site 1233 pre- sented in Lund et al. (this volume) document 5°–10° steeper inclina- tions in the cleaned U-channel paleomagnetic results. The inclination variability displays a strong sense of cyclicity overall. The most com- mon cyclicity occurs with a stratigraphic interval of ~0.5–1 m, but longer ~3-m cycles are also intermittently visible. The declinations typi- cally vary from B20° to +20°, with an artificially imposed long-term average declination near 0°. The declinations display the same overall pattern of cyclicity shown for the inclinations. Radiocarbon age determinations suggest that Site 1234 has an aver- age sediment accumulation rate of 0.5–1.0 m/k.y. in the uppermost 30 mcd. On that basis, inclination and declination cyclicity duration at Site 1234 ranges primarily from ~1000 to 3000 yr, very similar to values noted at Site 1233. Such values are also quite similar in amplitude and frequency to late Quaternary PSV observed in the Northern Hemisphere (e.g., Lund, 1993, 1996). The Site 1234 directions also display three short intervals of signifi- cantly higher variability: (1) 4.3–5, (2) 18–19, and (3) 22–23.5 mcd. The inclinations between 4.3 and 5 mcd vary dramatically in a nonrepro- ducible manner, even though the declinations appear to behave nor- mally. The NRM intensity in this interval is exceptionally weak (Mix, Tiedemann, Blum, et al., 2003), and we interpret these anomalous incli- nations to be the result of rock magnetic complexities, most probably the continuing presence of a relatively strong viscous (positive inclina- tion) overprint. The declinations between 18 and 19 mcd make a signif- icant swing to the east (80° in Hole 1234A and 50° in Hole 1234C) before returning to values near 0°, whereas the inclinations vary in a more normal manner. Taken by itself, we might consider this to be some type of sedimentologic artifact; however, this pattern is very simi- lar to the pattern shown at 57–59 mcd at Site 1233. Our correlations in- dicate that the overall pattern of variability surrounding 18–19 mcd in Site 1234 and 57–59 mcd in Site 1233 are correlative and likely repre- sent a geomagnetic excursion. The directions in the third anomalous interval from 22 to 23.5 mcd make an almost complete polarity reversal (Fig. F9). The inclinations in all three holes reach almost +50° (full re- F9. Site 1234 PSV records, 0–15 versal), but the declinations in the three holes somewhat disagree with mcd, p. 19.
A Composite Hole Hole Hole PSV Splice 1234A 1234B 1234C each other. Two of the declination records (Holes 1234B and 1234C) -40° -40° 0° -40° 0° -40° 0° ° 21 display an almost ±100 oscillation in declination around the imposed I36 ° 22 0 baseline, whereas the declinations in Hole 1234C make a complete I37
23 180° reversal. We believe that these records represent a second excur- Sediment depth (mcd) sion in the sequence, but there is significant residual overprint that pre- 24 B Composite Hole Hole Hole PSV Splice 1234A 1234B 1234C -100° 0°-100° 0° 0° 0° vents certainty in the exact pattern of excursional behavior. Detailed U- 21
D37' channel measurements will be required to better define the pattern of 22 directional variability in this interval. D38 23 Sediment depth (mcd) D39
24 Correlation of Paleomagnetic Secular Variation from Sites 1233 and 1234
The similar scale of cyclicity at Sites 1233 and 1234 and their proxim- ity (~500 km apart) both suggest that PSV correlations should be possi- ble. To begin the correlation, we assumed that the excursional declination swing near 58 mcd at Site 1233 (D44/D43/D42) can be cor- S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 6 related with the similar excursional declination swing near 18 mcd at Site 1234 (D34/D33/D32). Similarly, we assumed that the larger interval of excursional directions near 68 mcd at Site 1233 (D51/D50/D49, I56/ I55/I54) can be correlated with the second larger interval of excursional directions near 22.5 mcd at Site 1234 (D39/D38/D37, I38/I37/I36). Shipboard correlation of the Site 1233 paleomagnetic data and mag- netic susceptibility to its site survey Core GeoB-3313-1 (Mix, Tiede- mann, Blum, et al., 2003) and associated radiocarbon dates indicate that Site 1233 cores were recovered to within ~10 cm of the sediment/ water interface. Correlation of Site 1234 paleomagnetic data and mag- netic susceptibility to its site survey gravity Core RR9702A-41GC (S.P. Lund, unpubl. data) also indicate that the Site 1234 cores were recov- ered to within ~20 cm of the sediment/water interface. On that basis, we expected that the PSV in the uppermost 1–3 mcd at each site should generally correlate. The actual correlation attempted to match specific inclination and declination peaks or troughs in the two records using the general tie points previously identified as a starting point. In all cases, the phase re- lationships noted between inclination and declination peaks or troughs at each site had to be preserved in the correlation to the other site. Ulti- mately, a table of all correlatable PSV features was developed. This table is available at earth.usc.edu/~slund/data/index.php. The depths of correlatable PSV features at Sites 1233 and 1234 are plotted in Figure F10. It is clear that Site 1234 has an average sediment accumulation rate F10. Excursion 3β detail, Site 1234, in the uppermost 30 mcd that is only about one-third that of Site 1233. p. 20.
30 Chronostratigraphy of Sites 1233 and 1234 25 20 A total of 20 accelerator mass spectrometry (AMS) radiocarbon dates 15 10 have been recovered from Site 1233 (Lamy et al., 2004), and another Site 1234 sediment depth (mcd) 5 seven AMS radiocarbon dates have been recovered from site survey 0 0 1020304050607080 Core GeoB-3313-1 and correlated to Site 1233 (Lamy et al., 2002; Mix, Site 1233 sediment depth (mcd) Tiedemann, Blum, et al., 2003). All 27 dates are plotted in Figures F1, F2, F3, F4, and F5. Time-depth plots for Sites 1233 and 1234 were devel- oped using these dates (Fig. F11). These results suggest that the sedi- F11. PSV depth correspondence, ment accumulation rate at Site 1234 has been about one-third that of p. 21. Site 1233 for the last 45,000 yr. 50
Excursion 3β The stratigraphic locations of the most recent excursions from Site 40 Excursion 3α ) 3 1233 and Site 1234 are also noted by small boxes. The two excursions 30
Site occur at ~35,000 cal. yr before present (BP) and 41,000 cal. yr BP, both 1234 20
Calendar years BP (x 10 Calendar years Site within oxygen isotope Stage 3. On that basis, we have initially named 1233 10 them Excursion 3α and 3β, respectively. This is consistent with the
0 0 10 20 30 40 50 60 70 80 naming pattern developed by Lund et al. (2001) for the Brunhes-aged Depth (mcd) excursions for ODP Leg 172, named for the oxygen isotope stage within which they occur and the youngest excursion in each stage is labeled “α.” Lund et al. (1998, 2001) have argued that a global excursional state occurs intermittently during periods of stable magnetic polarity. During such states, it is common to have excursions occurring in many parts of the planet at essentially the same time, even though they may not have the same pattern of directional variability. Understanding the global space-time pattern of field behavior within and surrounding excur- sional states will be needed to assess how excursions from different parts of the planet relate to one another. Our new excursion records support that view. Excursion 3α at ~35,000 cal. yr BP is not significantly different in age from the Mono Lake excursion noted in western North S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 7
America (~32,000–34,000 cal. yr BP; Benson et al., 2003). Excursion 3β at ~41,000 cal. yr BP is not significantly different in age from the Las- champ excursion in the North Atlantic region, recently summarized by Lund et al. (in press), also at ~41,000 cal. yr BP. It is important to note, however, that the directional variability for Excursions 3α and 3β at Sites 1233 and 1234 is significantly different in pattern from that of the Mono Lake and Laschamp excursions in the Northern Hemisphere. Excursion 3α is very unusual in that its anoma- lous directions are limited to a large (~90°) easterly swing in declina- tion, unlike the oscillatory variation noted by Liddicoat and Coe (1979) at Mono Lake, California (USA). The directional morphology of Excur- sion 3β at Site 1233 is actually a full polarity reversal, whereas the Las- champ Excursion in the North Atlantic region (Lund et al., in press) has an oscillatory variability very similar to the Northern Hemisphere Mono Lake Excursion. We can also use the new chronology to plot time series of the overall PSV from the Site 1233 and 1234 splices for the last 50,000 yr, as shown in Figure F12. It is clear that there are a large number of correlatable di- F12. PSV time series, p. 22.
-80° rectional features between the two sites. Moreover, the PSV patterns Inclination
Site 1233 surrounding the two excursions are strikingly similar, corroborating our -60°
-40° α initial assertion that the two excursion records at Sites 1233 and 1234 β -60°
-40° Excursion 3 were the same. Even so, there are significant intervals in inclination, Excursion 3 -20° Site 1234 such as at ~5,000 and 15,000 cal. yr BP, where the records are quite dif- ferent even though the declinations in those intervals compare well. 40° Declination Site 1233
We attribute this to the presence of some residual positive inclination 0° α overprint that has not been removed. We expect that U-channel β 40° -40° Excursion 3 paleomagnetic studies, which will include more careful AF demagneti- Excursion 3 0° zation, will be necessary to recover more accurate PSV records from Site 1234
-40° 0 10 20 30 40 50 these two sites. Calendar years BP (x 103)
CONCLUSIONS
The shipboard paleomagnetic records from Sites 1233 (entire 135 mcd) and 1234 (uppermost 30 mcd) are reproducible among all the holes at each site and can be generally correlated between sites for the last ~50,000 yr. The PSV patterns at both sites display directional cyclic- ity in the ~700–3500 yr range, which is consistent with patterns observ- able in the Northern Hemisphere for the same time interval. The inclination records, especially at Site 1234, show evidence for the con- tinuing presence of some residual magnetic overprint (steep positive in- clination) after 25-mT AF demagnetization, which makes the average inclinations anomalously low. This overprint may also be the cause of poorer PSV correlations between Sites 1233 and 1234 at ~5,000 and ~15,000 cal. yr BP. A total of 27 AMS radiocarbon dates permitted us to develop time- depth curves for both sites over the last ~50,000 yr. The curves suggest that Site 1233 had sediment accumulation rates of ~1.5 m/k.y. for the last 50,000 yr, but the sediment accumulation rate at Site 1234 was only ~0.5 m/k.y. for the last 50,000 yr. Both sites contain reproducible and correlatable evidence for two ex- cursions at ~35,000 cal. yr BP (Excursion 3α) and ~41,000 cal. yr BP (Ex- cursion 3β). These two excursions are not significantly different in age from the Mono Lake and Laschamp Excursions, respectively, but they appear to have distinctly different patterns of directional variability at S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 8
Sites 1233 and 1234. We also note clear indication of a third, older ex- cursion at Site 1233, but we cannot yet date it with any certainty.
ACKNOWLEDGMENTS
This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Founda- tion (NSF) and participating countries under the management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by United States Science Support Program funds from NSF. S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 9 REFERENCES
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Figure F1. Paleomagnetic secular variation (PSV) records for 0–30 mcd in Holes 1233A–1233E. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. Cal. yr BP = calendar year before present. A. Inclination. I1–I28 = selected reproducible features. B. Declination. D1–D25 = selected reproducible features. A Composite PSV splice Hole 1233A Hole 1233B Hole 1233C Hole 1233D Hole 1233E -80° -60° -60° -60° -60° -60° -60° -40° -20° 0 I1 260 I2 I3 1,180 I4 1,980 I5 3,820 5 I6 I7 5,040 I8 I9 5,590
I10 7,090 I11 10 8,780 I12 I13 I14 10,040 I16 I15 15 I18 I17 I19 12,260 I20 I21 20 I22
Sediment depth (mcd) 13,620 I23 I23 ‘ I24 I25 25 I26 17,240 I27 19,680 I28 Cal. yr BP 30 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20°0° 20° 0° 20° 0° 20° 0° 20° 0° 20° 0 260 D2 D1 D3 1,180 D4 1,980 D5 3,820 5 D6 D7 5,040 D8 5,590 D9 D10 7,090 10 8,780 D11 10,040
15 D12 D13 D14 12,260 D15 D16 20 Sediment depth (mcd) 13,620 D17 D17 ‘ D18 25 D19 D20 17,240 D21 D22 19,680 D23 Cal. yr BP D24 30 D25 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 12
Figure F2. Paleomagnetic secular variation (PSV) records for 30–60 mcd in Holes 1233B–1233E. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. Cal. yr BP = calendar year before present. A. Inclination. I29–I50 = selected reproducible features. Features are normally noted in at least three individual holes; features shown in parentheses are only seen in two holes. B. Declination. D26–D44 = selected reproducible features. A Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -80°-60° -60° -60° -60° -60° -40° 30 I29 I30 ( I31 ) 21,480 ( I32 ) 35 I33 22,510 I34
I35 40 23,860 I36 I37 I38
26,050 I39 45
I39 ‘ 28,830 I40 29,510 I41
Sediment depth (mcd) 50 ( I42 ) 29,780 ( I43 ) ( I44 ) ( I44 ‘) 55 I45 I46 32,340 I47 I48 I49 36,360 I50 Cal. yr BP 60 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20° 0° 20° 0°20° 0° 20° 0° 20° 0° 20° 40° 60° 80° 30 D26
D27 21,480
35 D28 D29 22,510
D30 40 D31 23,860 D32
D33 26,050 45 D34 D35 D35 ‘ 28,830 D36 29,510 D37 Sediment depth (mcd) 50 D38 29,780 D39 Interval of more uncertain D40 correlations 55 D41 32,340 D41 ‘ D42 D43 Excursion 3α 36,360 D44 Cal. yr BP 60 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 13
Figure F3. Paleomagnetic secular variation (PSV) records for 60–90 mcd in Holes 1233B–1233E. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. Features are nor- mally noted in at least three individual holes; features shown in parentheses are only seen in two holes. See Figure F6, p. 16, for detailed records for Excursion 3β. A. Inclination. I51–I78 = selected reproducible fea- tures. B. Declination. D45–D71 = selected reproducible features. A Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -80° -60°-60° -60° -60° -60° -40° 60 I51
I52
65 I53 (I54)
I55 Excursion 3β
70 (I56) I57 I58 I59 I60 75 I61 I62 I63 I64 I65 I66 I67
Sediment depth (mcd) 80 (I68) (I68) (I69) (I70) I71 I72 I73 85 I74 I75 I76 I78 I77 90 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20°0° 20° 0°20° 0° 20° 0° 20° 0° 20° 60 D45 D46 D47 D47 ‘ 65 D48 D49
D50 Excursion 3β
70 D51 D52 D53 D54 D55 D56 75 D57 D58 D59 D59 ‘ D60
Sediment depth (mcd) 80 D62 D61 D63 (D63?) (D64?) D65 Interval of 85 D66 more uncertain D67 correlations D68 D69 D70 90 D71 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 14
Figure F4. Paleomagnetic secular variation (PSV) records for 90–120 mcd in Holes 1233B–1233E. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. Features are nor- mally noted in at least three individual holes; features shown in parentheses are only seen in two holes. A. Inclination. I79–I94 = selected reproducible features. B. Declination. D72–D98 = selected reproducible features. A Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -60°-30° -60° -60° -60° -60° -30°0° 30° 90 I80 I79' (I81)
(I82) Zone of 95 questionable but reproducible ?I83? directions
I84 100 I85 I86
Intervals of 105 magnetic mineral dissolution I87 I88 I89 110 I90 Sediment depth (mcd) (I91) Unnamed excursion (I92) Zone of 115 unreproducible inclinations
(I94)
120 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D Hole 1233E -20°0° 20° 0°20° 0° 20° 0° 20° 0° 20° 40° 60° 90 D72 (D73) (D74) 95 (D75) D76 D77 D78 D79 D80 100 D81 D82 D83 Intervals of (D84) magnetic mineral D85 105 D85 ‘ dissolution D86 D87 D87 ‘ D88 110 D89 Sediment depth (mcd) D90 Unnamed excursion D91 D92 (D93) (D94) 115 (D95) (D96) (D97) (D98) 120 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 15
Figure F5. Paleomagnetic secular variation (PSV) records for 120–140 mcd in Holes 1233B–1233D. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. Features are nor- mally noted in at least three individual holes; features shown in parentheses are only seen in two holes. A. Inclination. I95–I106 = selected reproducible features. B. Declination. D99–D111 = selected reproducible features. ??? = questionable features, which are not reproduced in multiple holes. A Composite PSV splice Hole 1233D Hole 1233C Hole 1233C Hole 1233B -60°-30° -60° -60° -30° 0°30° 0° 30° 120 (I95) (I96)
(I97)
125 ???
(I98) (I99) (I100)
(I101) 130 (I102) (I103) (I104)
Sediment depth (mcd) (I105) (I106) 135
140 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D -20°0° 0° 0° 0° 20° 120 (D99) (D100)
(D101)
(D102) 125 ??? (D103) (D104) (D105) (D106) (D107) 130 (D108)
(D109)
(D110) Sediment depth (mcd) (D111) 135
140 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 16
Figure F6. Detailed paleomagnetic secular variation (PSV) records for Excursion 3β at Site 1233 (see Fig. F3, p. 13). Features are normally noted in at least three individual holes; features shown in parentheses are only seen in two holes. A. Inclination. I53–I56 = selected reproducible features. B. Declination. D47–D52 = selected reproducible features. A Composite PSV splice Hole 1233C Hole 1233D -60° -20° -60° -20° -60° -20° 64
65 (I53)
66 (I54)
67
(I55) 68
Sediment depth (mcd) 69
(I56) 70
71 B Composite PSV splice Hole 1233B Hole 1233C Hole 1233D -50° 0° -50° 0° -50° -0° -50° 0° 64 D47'
65 (D48) (D49) 66
67
68 D50
Sediment depth (mcd) 69
(D51) 70 D52
71 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 17
Figure F7. Paleomagnetic secular variation (PSV) records for 0–15 mcd in Holes 1234A–1234C. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. A. Inclination. I1–I26 = selected reproducible features. Features are normally noted in at least three individual holes; features shown in parentheses are only seen in two holes. B. Declination. D1–D26 = selected reproducible features. A Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -60°-40° -40° -40° -40° -20° 0 (I1) (I2) I3 I3'' I4 I5 I6 I7 I8 Interval of anomalously low inclinations 5 I10 I9 I10' I12 I11 I13 I14 I15 I16 I16 ‘ I17 I17 ‘ I18 10 I18' Sediment depth (mcd) I19 I20 I21 I22 I22 ‘ I23 I24 I25 I26 15 B Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -20°0° 20°0° 20° 0° 20° 0° 20° 0 D2 D1 D3 D3' D3'' D4 D5
D6 D7 5 D8 D9 D10 D10 ‘ D12 D11 D13 D14 D13 ‘ D15 D16
10 D18 D17
Sediment depth (mcd) D18 ‘ D19 D20 D21
D22 D24 D23 D25 15 D26 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 18
Figure F8. Paleomagnetic secular variation (PSV) records for 15–30 mcd in Holes 1234A–1234C. Small dots = discrete measurements at 5-cm intervals, solid lines = seven-point running average. See Figure F9, p. 19, for detailed records for Excursion 3β. A. Inclination. I27–I45 = selected reproducible features. B. Dec- lination. D27–D47 = selected reproducible features. A Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -60°-40° -40° -40° -40° -20° 0° 15 I27 I28 I29 I30
I31 I32 I33 20 I34
I35 I36 I37 Excursion 3β
I38
Sediment depth (mcd) 25 I39 I40 I41 I42 I43
I44 I45 30 B Composite PSV splice Hole 1234A Hole 1234B Hole 1234C -30°0° 0° 0° 0° 30° 15 D28 D27 D29 D30 D31 D32 D33 D34 Excursion 3α D35 20 D36 D37 D37'
D38 Excursion 3β D39 D40 D41 25 D42 Sediment depth (mcd) D43
D44
D45
D46 30 D47 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 19
Figure F9. Detailed paleomagnetic secular variation (PSV) records for Excursion 3β at Site 1234 (see Fig. F7, p. 17). A. Inclination. I36 and I37 = selected reproducible features. B. Declination. D37–D39 = selected re- producible features. A Composite Hole Hole Hole PSV Splice 1234A 1234B 1234C -40° -40° 0° -40° 0° -40° 0° 21
I36
22
I37
23 Sediment depth (mcd)
24 B Composite Hole Hole Hole PSV Splice 1234A 1234B 1234C -100° 0°-100° 0° 0° 0° 21
D37'
22
D38
23 Sediment depth (mcd) D39
24 S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 20
Figure F10. Depth correspondence for PSV features correlatable between Sites 1233 and 1234.
30
25
20
15
10 Site 1234 sediment depth (mcd)
5
0 0 1020304050607080
Site 1233 sediment depth (mcd) S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 21
Figure F11. Time-depth plot based on 27 AMS radiocarbon dates (solid dots) from Site 1233 that were trans- ferred to Site 1234 depths based on depth correlation curve in Figure F10, p. 20. Squares = depth interval for excursions at each site. BP = before present.
50
Excursion 3β 40
Excursion 3α ) 3
30
Site 1234 20
Calendar years BP (x 10 Calendar years Site 1233
10
0 0 10 20 30 40 50 60 70 80 Depth (mcd) S.P. LUND ET AL. PALEOMAGNETIC SECULAR VARIATION AND CHRONOSTRATIGRAPHY 22
Figure F12. PSV time series from Sites 1233 and 1234 for the last 50,000 yr. Selected correlatable inclination and declination features are indicated by vertical lines. BP = before present.
-80° Inclination
Site 1233 -60°
-40° α β -60°
-40° Excursion 3 Excursion 3
-20° Site 1234
40°
Declination Site 1233
0° α β 40° -40° Excursion 3 Excursion 3
0°
Site 1234
-40° 0 10 20 30 40 50 Calendar years BP (x 103)