# Age and duration of Laschamp and Iceland Basin geomagnetic excursions in the South Atlantic Ocean

James E. T. Channell, Natalia Vázquez Riveiros, Julia Gottschalk, C.

Waelbroeck, Luke C. Skinner

► To cite this version:

James E. T. Channell, Natalia Vázquez Riveiros, Julia Gottschalk, C. Waelbroeck, Luke C. Skinner. Age and duration of Laschamp and Iceland Basin geomagnetic excursions in the South Atlantic Ocean. Quaternary Science Reviews, 2017, 167 (3), pp.1 - 13. 10.1016/j.quascirev.2017.04.020. hal-01584125

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1 Age and duration of Laschamp and Iceland Basin geomagnetic excursions in the South

2 Atlantic Ocean

3

4 J.E.T. Channell¹, N. Vázquez Riveiros²,³, J. Gottschalk³,⁴, C. Waelbroeck², L.C. Skinner³

5

6 ¹Department of Geological Sciences, POB 112120, University of Florida, Gainesville, FL 32611,
7 USA

8 ²Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CNRS-CEA-UVSQ,
9 Université de Paris-Saclay, F-91198 Gif-sur-Yvette, France

10 ³Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of
11 Cambridge, Cambridge CB2 3EQ, UK

12 ⁴Institute of Geological Sciences and Oeschger Centre for Climate Change Research, University
13 of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland

14

15 Corresponding Author: J.E.T. Channell (jetc@ufl.edu, Tel: 352 392-3658, Fax: 352 392-9294)

16

17 Abstract

18 Age models for new records of the Laschamp and Iceland Basin excursions from the eastern
19 flank of the South Atlantic mid-ocean ridge (44.15°S, 14.22°W) are derived from radiocarbon
20 dates, and from matching sea-surface temperature records to Antarctic (EPICA) air-temperature
21 records from ice cores. The onset of the Laschamp excursion occurred during Antarctic Isotopic
22 Maximum (AIM) 10, consistent with its occurrence during Greenland Interstadial 10. The end of
23 the Laschamp excursion occurred prior to AIM 9 in Greenland Stadial 10. The age model is
supported by synchroneity of directional and relative paleointensity manifestations of the
Laschamp excursion in the marine core with peaks in EPICA$^{10}$Be and nitrate flux. The Iceland
Basin excursion is synchronous with the final phase of the transition from marine isotope stage
(MIS) 7a to MIS 6e as recorded in the EPICA δD record. The onset of the Laschamp and Iceland
Basin excursions, defined here by component inclinations >-40°, occurred at 41.4 ka and 190.0
ka, and durations are ~1 kyr and ~ 3.5 kyr, respectively, although these estimates depend on the
criteria used to define the directional excursions. By comparison with Laschamp and Iceland
Basin excursion records from the North Atlantic Ocean, the two excursions are synchronous at
centennial timescales between the two hemispheres, based on synchronization of the GICC05
and AICC2012 age models for Greenland and Antarctic ice cores.

34

35

# 1. Introduction

Documenting the spatial distribution of magnetic excursion records, and determining their
timing and duration, are important not only for gauging the usefulness of excursions in high-
resolution stratigraphy but also for understanding the workings of the geodynamo. The proposal
that the age of the Laschamp excursion, and its associated relative paleointensity (RPI)
minimum, is location dependent (Leonhardt et al., 2009), and that the Matuyama-Brunhes (M-B)
boundary is asynchronous from the Atlantic to the Pacific by ~10 kyr (Leonhardt and Fabian,
2007), have important implications for magnetic stratigraphy. Most Quaternary magnetic
excursion and RPI records are from north of 35°N in the North Atlantic Ocean. Southern
hemisphere records of the Laschamp excursion have been reported from several piston cores
from the vicinity of the South Atlantic Agulhas Ridge (Fig. 1, Channell et al., 2000), from two
piston cores from the Scotia Sea (Collins et al., 2012), from a single Indian Ocean core (MD94-
103) collected east of the Kerguelen Plateau (Mazaud et al., 2002), and from several sites
occupied during Ocean Drilling Program (ODP) Leg 202, principally Site 1233 at 41.0°S,
74.26°W, off southern Chile (Lund et al., 2006a,b; Stoner et al., 2008). The only southern
hemisphere record of the Iceland Basin excursion is from ODP Site 1089, again from the
Agulhas Ridge (Fig. 1, Stoner et al., 2003).

Age models for the Agulhas Ridge records of the Laschamp and Iceland Basin excursions
were based on benthic oxygen isotopes (Channell et al., 2000; Hodell et al., 2001; Mortyn et al.,
2003) and their correlation to nearby Core RC11-83, which has 14 radiocarbon ages in the 11-41
ka interval (Charles et al., 1996). The age models for the Scotia Sea Laschamp records (Collins
et al., 2012) were derived by RPI correlation to the South Atlantic (SAPIS) paleointensity stack
(Stoner et al., 2002). Similarly, the age model for the Indian Ocean record (Mazaud et al., 2002)
was based on correlation of RPI proxies to the North Atlantic (NAPIS) RPI stack (Laj et al.,
2000). Finally, the age model for the ODP Site 1233 (Lamy et al., 2004) was determined by
correlation of alkenone sea-surface temperature (SST) data to the Byrd (Antarctic) ice core δ¹⁸O
record of Blunier and Brook (2001).

Here we report records of the Laschamp and Iceland Basin excursions from cores MD07-
3076Q and MD07-3077, collected from *R/V Marion Dufresne* at a site (44.15°S, 14.22°W) on
the eastern flank of the mid-Atlantic ridge at a water depth of 3770 m. The location is ~1985 km
west of Agulhas Ridge (Fig. 1). Core MD07-3076Q is a 10.9-m square-section (25 x 25 cm)
gravity core, and core MD07-3077 is a 49.5-m *Calypso* piston core that extends back through the
last ~500 kyr. The top ~10-15 m of cores retrieved with the *Calypso* corer prior to 2016 are
usually “oversampled” (stretched) by ~30-40% during recovery (see Skinner and McCave, 2003;
Széréméta et al., 2004). For this reason, we used the top six sections (sections are usually 1.5-m
in length) of core MD07-3076Q down to 8.20 meters below seafloor (mbsf), which is equivalent to 75 ka, and core sections 9 to 21 of core MD07-3077 that correspond to the 12 to 31 mbsf interval in this core. Below 31 mbsf (~250 ka), low magnetization intensities limit the fidelity of the records, so we restrict our discussion to the last 250 kyr. Blue reflectance data from the two cores (Fig. 2) indicate that the base of the studied interval in core MD07-3076Q (at 8.2 mbsf) corresponds to the 13.5 mbsf level in core MD07-3077 due to core stretching in the upper part of core MD07-3077.

The new South Atlantic records of the Laschamp and Iceland Basin excursions are notable, not only because southern hemisphere excursion records are rare, but also because of their relatively enhanced age control based on calibrated radiocarbon ages and on correlation of sea surface temperature proxies to the EPICA Dome C (EDC) δD record. The age control enables us to link the South Atlantic records of the two excursions to Antarctic (and Greenland) ice core chronologies, and hence to North Atlantic records of the same excursions, and to assess the global synchroneity of these magnetic excursions and their utility as stratigraphic markers.

## 2. Age Model

The age model for core MD07-3076Q back to 27 ka is based on 50 calibrated accelerator mass spectrometer (AMS) $^{14}$C ages from monospecific planktic foraminifera, which are corrected for variable reservoir age effects based on alignment of variations in sea surface temperature to changes in Antarctic air-temperature from ice-core records (Skinner et al., 2010; Table 1; Fig. 2). Sea surface temperatures were reconstructed based on Mg/Ca ratios of planktic foraminifera *Globigerina bulloides* and *Neogloboquadrina pachyderma* (sin.), and on abundance variations of *N. pachyderma* (sin.) (Skinner et al., 2010; Vázquez Riveiros et al., 2010).
During marine isotope stage (MIS) 3, beyond 27 ka, the age model for core MD07-3076Q (Table 1, Fig. 2) is based on the stratigraphic alignment of abundance peaks of *G. bulloides* with Antarctic air-temperature maxima represented by minima in the EDC δD Antarctic ice-core record (Jouzel et al., 2007), transferred onto the AICC2012 age scale (Bazin et al., 2013; Veres et al., 2013). This age model (Table 1, Fig. 2) is consistent within 330 ±280 years with previously established chronologies for core MD07-3076Q (Gottschalk et al., 2015a,b).

A similar strategy was used for core MD07-3077 utilizing the alignment of abundance peaks of *G. bulloides* and abundance lows of *N. pachyderma* (sin.), with Antarctic air-temperature maxima represented by minima in the EDC δD Antarctic ice-core record (Fig. 2), following the procedure used for MIS 11 in the same core (Vázquez Riveiros et al., 2010; 2013). Two additional tie points for core MD07-3077, based on the correlation of maxima in planktic δ¹⁸O with minima in Antarctic air-temperature, were added in MIS 4 (Table 2). Correlation between cores MD07-3076Q and MD07-3077 down to 13.5 mbsf (~100 ka) in MD07-3077 is based on blue reflectance data from each core (Fig. 2e), which permits the age model of MD07-3076Q to be transferred to MD07-3077, where the two cores overlap.

### 3. Natural Remanent Magnetization (NRM)

Continuous u-channel samples (2 x 2 x 150 cm³ samples encased in plastic with a clip-on lid constituting one of the sides) were collected from slice B of the square-section gravity core MD07-3076Q (Sections 1-6) and from the archive halves of Sections 9-33 of calypso core MD07-3077. Sections 1-8 of core MD07-3077 were not sampled due to evidence for core stretching in the upper part of core MD07-3077, and because this part of the core was recovered in better condition in core MD07-3076Q. Measurements of the natural remanent magnetization
116 (NRM) of u-channel samples were made at 1-cm intervals, with a 10-cm leader and trailer at the
117 top and base of each u-channel sample, using a 2-G Enterprises pass-through magnetometer at
118 the University of Florida designed for the measurement of u-channel samples (Weeks et al.,
119 1993; Guyodo et al., 2002). After initial NRM measurement of u-channel samples, stepwise
120 alternating field (AF) demagnetization was carried out in 5 mT increments in the 10-60 mT peak
121 field interval, and in 10 mT increments in the 60-100 mT interval, using tracking speeds of 10
122 cm/s. Component magnetizations were computed each 1-cm for a uniform 20-80 mT
demagnetization interval (Fig. 3) following the standard least-squares method (Kirschvink, 1980)
123 without anchoring to the origin of orthogonal projections, using UPmag software (Xuan and
124 Channell, 2009). Maximum angular deviation (MAD) values are generally <10°, indicating
125 moderately well-defined magnetization components apart from an interval close to the base of
126 MD07-3076Q, and the lower half of MD07-3077 below ~35 mbsf (Fig. 3). Component
127 declinations in Figure 3 are arbitrary and have not been adjusted for vertical-axis core rotation.
128 The declination should, however, be uniform for the entire core because a consistent split-face
129 for each core section was sampled. Twisting of sediment core is apparent in the declination
130 record of core MD07-3077 (Fig. 3), particularly in the 25-35 mbsf (175-280 ka) interval.
131 Although core twisting affects the component declinations, two magnetic excursions are apparent
132 in the u-channel inclination and declination records at 4.06-4.36 mbsf in core MD07-3076Q, and
133 at 26.48-27.00 mbsf in core MD07-3077 (Fig. 3). As will be discussed below, the former is a
134 record of the Laschamp excursion (~41 ka) and the latter is a record of the Iceland Basin
135 excursion (~188 ka). Below ~32 mbsf, MAD values increase as NRM intensities decrease (Fig.
136 3). We associate this decrease in magnetization intensity with magnetite dissolution associated
137
138 with pore-water sulfate reduction. For this reason, data from below ~32 mbsf (250 ka) are not discussed further.

140

141 **4. Laschamp and Iceland Basin excursions**

142 Sedimentary records of the Laschamp excursion (~41 ka) are widely distributed in the
143 North Atlantic Ocean and Gulf of Mexico (e.g., Laj et al., 2000, 2006; Lund et al., 2005;
144 Channell, 2006; Evans et al., 2007; Channell et al., 2012), with additional records from the Black
145 Sea (Nowaczyk et al., 2013), southern Indian Ocean (Mazaud et al., 2002), the Agulhas Ridge in
146 the South Atlantic Ocean (Channell et al., 2000), Scotia Sea (Collins et al., 2012), and ODP Site
147 1233 off southern Chile (Lamy et al., 2004; Lund et al., 2006a,b; Stoner et al., 2008).

148 The Iceland Basin excursion (~190 ka) has been recorded repeatedly in sediments from the
149 North Atlantic Ocean (e.g., Channell et al., 1997; Channell, 1999, 2006, 2014; Laj et al., 2006;
150 Evans et al., 2007), the Pacific Ocean (e.g., Yamazaki and Ioka, 1994; Roberts et al., 1997; Laj et
151 al., 2006), and from Lake Baikal (Oda et al., 2002). Until now, the only record of the Iceland
152 Basin excursion reported from the southern hemisphere is from ODP Site 1089 from the Agulhas
153 Ridge (Stoner et al., 2003).

154 NRM component directions from u-channel samples from cores MD07-3076Q and MD07-3077, although distorted by core twisting, indicate the presence of two magnetic excursions, at
155 ~4 mbsf in core MD07-3776Q and at ~26.5 mbsf in core MD07-3077 (Fig. 3). The respective
156 age models place the excursions at ~41 ka and ~190 ka, which indicates that they represent the
157 Laschamp and Iceland Basin excursions, respectively. In order to confirm the presence of
158 magnetic excursions in cores MD07-3076Q and MD07-3077, we collected cubic discrete (2 x 2
159 x 2 cm³) samples back-to-back alongside the u-channel trough. Stepwise AF demagnetization of
160
discrete samples resulted in magnetization components, calculated for a uniform 20-80 mT demagnetization interval, that indicate that the Laschamp and Iceland Basin excursions are recorded by both discrete and u-channel samples (Fig. 4). MAD values are, however, >10° for some discrete and u-channel measurements from the excursion intervals (Fig. 4), indicating poorly-defined magnetization components. A uniform 20-80 mT demagnetization interval for calculation of component magnetizations, for both discrete and u-channel samples, allows unambiguous assessment of data quality. Orthogonal projections of AF demagnetization data for discrete and u-channel samples that record the excursions often display well-defined magnetization components (Fig. 5), although component inclinations do not reach high positive values for either excursion, and the declination change for the Laschamp excursion is muted (Fig. 4). By comparison with numerous excursion records from the North Atlantic Ocean (e.g., Laj et al., 2006; Channell et al., 2012), the lack of antipodal directions from discrete and u-channel samples indicates that the NRM fidelity is compromised by magnetite dissolution. The median destructive field (MDF) of NRM increases abruptly from ~27 mT to ~45 mT in the 32-38 mbsf interval, implying the authigenic growth of high coercivity iron sulfides below ~35 mbsf.

## 5. Relative Paleointensity (RPI)

The intensity of detrital remanent magnetization (DRM) depends on the intensity of the geomagnetic field, and the concentration and alignment efficiency of remanence-carrying grains. The relative strength of the magnetizing field can be determined by using the intensity of different types of laboratory-induced magnetizations, including anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM), to normalize the NRM
184 intensity for changes in concentration of remanence-carrying grains. The normalizer should,
185 therefore, activate the same grains that carry the NRM. The resulting normalized remanence can
186 be used as a proxy for RPI variations if magnetite is the sole NRM carrier and occurs in a
187 restricted grain-size range (Banerjee and Mellema, 1974; Levi and Banerjee, 1976; King et al.,
188 1983; Tauxe, 1993).

189 After demagnetization and analysis of the NRM, ARM was imposed along the long-axis
190 of u-channel samples in a peak AF of 100 mT and a DC bias field of 50 µT, and was then
191 demagnetized at the same peak fields used to demagnetize the NRM. Subsequently, stepwise
192 ARM acquisition was carried out using a uniform bias field (50 µT) and increasing values of
193 peak AF (ARMAQ) at the same steps as for stepwise demagnetization. IRM₀.₃T was acquired
194 using impulse fields of 0.3 T, and was demagnetized at the same peak fields as applied to the
195 NRM and ARM, and then an additional IRM₁T, acquired in impulse fields of 1T, was
196 demagnetized once more at the same peak demagnetization fields. This RPI protocol (Channell
197 et al., 2002) allows us to calculate four RPI proxies as slopes: NRM/ARM, NRM/ARMAQ,
198 NRM/ IRM₀.₃T, and NRM/ IRM₁T; all calculated for the 20-60 mT demagnetization or
199 acquisition interval using the UPmag software of Xuan and Channell (2009). Linear correlation
200 coefficients (r) associated with each slope indicate that the slopes are well defined with r-values
201 >0.98 (Fig. 6). The RPI proxies are essentially identical for NRM/ARM and NRM/ARMAQ (as
202 expected), and for NRM/ IRM₀.₃T, and NRM/ IRM₁T, but are different for NRM/ARM versus
203 NRM/IRM particularly for 0-20 ka (Fig. 6). The RPI proxies can be partially matched to
204 reference RPI records (Fig. 6) such as the South Atlantic paleointensity stack (SAPIS, Stoner et
205 al., 2002), and to the global PISO paleointensity stack (Channell et al., 2009). There are,
206 however, notable differences between the new RPI records and the two reference stacks (Fig. 6),
particularly in the 0-30 ka and 60-80 ka intervals, and in the timing of RPI minima at ~115 ka and ~190 ka which correspond to the Blake and Iceland Basin excursions. For ages >250 ka (below ~35 mbsf), the amplitude of RPI proxies is subdued relative to the PISO reference record.

210

## 6. Rock magnetism

In addition to data acquired for NRM and RPI investigations, low-field volume susceptibility (κ) was measured at 1-cm intervals using a susceptibility track designed for u-channel samples that has a Gaussian-shaped response function, with width at half height of ~4 cm, similar to the response function of the u-channel magnetometer (Thomas et al., 2003).

Following King et al. (1983), the ratio of anhysteretic susceptibility (κARM, ARM intensity divided by the DC bias field used to acquire the ARM) to susceptibility (κ) can be used to estimate the “average” or bulk grain size of magnetite. This ratio for cores MD07-3076Q and MD07-3077 has a wide range of values (Fig. 7a). Ratios corresponding to bulk grain sizes <1 µm are from the top 1.3 m in core MD07-3076Q (purple triangles in Fig. 7a).

Additional magnetic mineralogical information was obtained for magnetic hysteresis parameters measured on a Princeton Measurements Corp. vibrating sample magnetometer (VSM). Hysteresis ratios: M<sub>rs</sub>/M<sub>s</sub> and B<sub>cr</sub>/B<sub>c</sub> where M<sub>rs</sub> is the saturation remanence, M<sub>s</sub> is the saturation magnetization, B<sub>cr</sub> is the coercivity of remanence, and B<sub>c</sub> is the coercive force, can be used to delineate single domain (SD), pseudo-single domain (PSD) and multidomain (MD) magnetite and to assign bulk magnetite grain sizes through empirical and theoretical calibrations of the so-called Day plot (Day et al., 1977; Carter-Stiglitz et al., 2001; Dunlop, 2002; Dunlop and Carter-Stiglitz, 2006). The Day-plot for samples from cores MD07-3076Q and MD07-3077, by comparison with measurements of unannealed sized magnetites (Dunlop, 2002), is consistent
230 with the presence of magnetite with bulk grain sizes in the 0.1-3 µm range (Fig. 7b), which is
231 broadly consistent with the κARM versus κ plot (Fig. 7a). Note that these “average” (bulk)
232 magnetite grain size estimates are function of grain size mixing over an undetermined grain-size
233 range.

234 Magnetic hysteresis properties were also analyzed using first-order reversal curves (FORCs)
235 that provide enhanced mineral and domain state discrimination (Pike et al., 1999; Roberts et al.,
236 2000; Muxworthy and Roberts, 2007). FORCs are measured by progressively saturating a small
237 (few hundred mg) sample in a field (Bsat), decreasing the field to Ba, reversing the field and
238 sweeping it back to Bsat in a series of regular field steps (Bb). The process is repeated for many
239 values of Ba. The magnetization is then represented as a contour plot with axes Bc and Bu where
240 Bc=(Bb-Ba)/2 and Bu=(Bb+Ba)/2. The contoured FORC distribution can be interpreted in terms of
241 the coercivity distribution along the Bc axis, and spreading of the distribution along the Bu axis
242 provides a measure of magnetostatic interactions for SD grains or internal demagnetizing fields
243 for MD grains. The latter dominates in weakly magnetized deep-sea sediments, and spreading in
244 Bu combined with low Bc can be interpreted in terms of high MD magnetite content. In general,
245 closed peaked structures along the Bc axis are characteristic of SD grains, with contours
246 becoming progressively more parallel to the Bu axis with grain-size coarsening. FORC diagrams
247 were analyzed using the software of Harrison and Feinberg (2008) with a smoothing factor (SF)
248 of 6 for a protocol using an averaging time of 1 s and a field increment of 2 mT up to a
249 maximum applied field of 1 T.

250 FORC diagrams from the uppermost sediments of core MD07-3076Q, corresponding to high
251 κARM/κ values (fine magnetite grain sizes), are characterized by a pronounced ridge along the Bc
252 axis and low dispersion along the Bu axis (Fig. 7c). This pattern implies weak magnetostatic
interactions and dispersed fine-grained biogenic magnetite (see examples in Egli et al., 2010; Roberts et al., 2011, 2012; Yamazaki, 2012; Channell et al., 2013). In contrast, FORC diagrams from below ~1.3 mbsf have lower coercivities ($B_c$) and more divergent FORC distributions along the $B_u$ axis, which is indicative of the increased abundance of coarser magnetite (Fig. 7d).

## 7. Transmission Electron Microscopy

For transmission electron microscope (TEM) imaging, magnetic extracts were prepared from MD07-3076Q at 0.86-0.91 mbsf (~14 ka) and 2.20-2.25 mbsf (~30 ka). The ~20 $cm^3$ sediment samples were sonicated in a sodium metaphosphate dispersant. The solutions were loaded into a reservoir that feeds a circulating system driven by a peristaltic pump that allows the fluid to pass slowly, without turbulence, past the outside of a test-tube containing a rare-earth magnet. The material that adhered to the outside of the test-tube was then removed to a methanol solution using a methanol squeeze-bottle. Grains of magnetic separate were adhered to a 3-mm copper TEM grid using another magnet suspended a few cm above the floating grid (see Chang et al., 2012).

Magnetic particles were imaged using a JEOL JEM-2010F high-resolution (HR) TEM in conjunction with energy dispersive x-ray spectroscopy (EDS) at an accelerating voltage of 200 kV. The microscope is equipped with a GatanMultiScan Camera Model 794 for imaging and an Oxford Instruments detector with INCA 4.05 software for microanalysis. Spot analysis and line-scans were conducted in scanning TEM (STEM) mode with a nominal ~1 nm probe size and a camera length of 12 cm.

In the sample from ~0.86-0.91 mbsf (~14 ka), euhedral grains, generally smaller than 100 nm across, but occasionally reaching 200 nm, were observed (Fig. 8 a,b). The shape and size of
276 these grains is consistent with them being magnetosomes produced by magnetotactic bacteria
277 (see Kopp and Kirschvink, 2008; Egli et al., 2010, Roberts et al., 2011, 2012; Yamazaki, 2012).

278 Magnetite octahedra (rectangular shapes with flattened corners) and arrowhead shapes (Fig. 8
279 a,b) are reminiscent of shapes of bacterial magnetite observed off the SW Iberian Margin
280 (Channell et al., 2013) and elsewhere (Kopp and Kirschvink, 2008). In the sample from 2.20-

281 2.25 mbsf (~30 ka), the magnetite population is apparently different, with rare bacterial
282 magnetite grains but abundant larger irregular-shaped detrital magnetite grains (Fig. 8 c). EDS

283 elemental analyses indicate that the irregular-shaped (detrital) grains contain Fe, O, and Ti, (Fig.
284 8 b,c) whereas the finer euhedral (biogenic) magnetites contain Fe and O, but no detectable Ti
285 (Fig. 8 b). The lack of Ti in magnetosome magnetite provides a well-documented means (besides

286 grain shape and size) of distinguishing detrital and biogenic magnetite. The predominance of
287 ultra-fine biogenic magnetite in the sample from ~0.86-0.91 mbsf (~14 ka) is consistent with the
288 existence of ultra-fine magnetite in the Holocene of core MD07-3076Q (Fig. 7a, c).

289

290 **8. Discussion**

291 The ages of onset and cessation of the Laschamp directional excursion at 41.4 ka and 40.4
292 ka (Fig. 9) are consistent, at centennial scale, with the Laschamp age and duration determined by
293 correlation of the δ¹⁸O record from North Atlantic Ocean core PS2644-5 (Voelker et al., 1998) to
294 the GISP2 ice core (Laj et al., 2000), and with the GICC05 age (41.25 ±0.8 ka) of the center of
295 the ¹⁰Be maximum that corresponds to the Laschamp excursion in the GRIP ice core (Yiou et al.,
296 1997; Svensson et al., 2006, 2008). Our South Atlantic result is also consistent with the mid-
297 point age of 41.3 ± 0.6 ka for the Laschamp excursion (Laj et al., 2014) based mainly on
298 ⁴⁰Ar/³⁹Ar age determinations from the Chaines des Puys (France), assuming an Alder Creek
299 (AC) rhyolite standard age of 1.193 Ma, which is equivalent to 28.02 Ma for the Fish Canyon
300 (FC) sanidine standard advocated by Renne et al. (1998). If more recent estimates for the ages of
301 the AC and FC standards are used (1.2061 Ma and 28.305 Ma; Renne et al., 2010), then the 41.3
302 ka age for the Laschamp excursion increases to 42.0 ka, which is just outside our estimate based
303 on correlation to the EDC ice core (Fig. 9). This discrepancy may imply issues with the newer
304 standard ages, as observed for the M-B boundary (Channell et al., 2010).

305 Our Laschamp age estimate is consistent with U-Th dating of a North American speleothem
306 that records of the Laschamp excursion at 41.1 ±0.35 ka (Lascu et al., 2016), although their
307 estimated duration for the directional excursion (2.5 kyr) is 2.5 times greater than estimated here.
308 The Laschamp record from ODP Site 1233 off Chile yields an excursion age centered at 41.0 ka,
309 and a duration for southern hemisphere VGPs of ~600 yr (Stoner et al., 2008), lower than our
310 estimate.

311 From correlation of MD07-3076Q SST data to the EDC δD record, the onset of the
312 Laschamp excursion coincides with Antarctic Isotopic Maximum (AIM) 10 warm episode (Fig.
313 9), and hence with Greenland Interstadial (GI) 10 according to inter-hemispheric correlations
314 (Veres et al., 2013). The end of the Laschamp excursion appears to coincide with AIM 9 and
315 Greenland Stadial (GS) 10. In the Antarctic EDC ice core, a broad irregular ¹⁰Be flux peak in the
316 39.5 ka to 42.5 ka interval (AICC2012 age) is accompanied by a distinct nitrate flux peak
317 centered at 41.2 ka (Fig. 9e, Traversi et al., 2016). Nitrate and ¹⁰Be fluxes depend partially on the
318 intensity of cosmic ray flux into Earth's upper atmosphere, and are therefore modulated by
319 geomagnetic field intensity. The EDC nitrate peak is apparently closely synchronous with the
320 Laschamp directional excursion recorded in core MD07-3076Q. The EDC ¹⁰Be peak is broader
than the nitrate peak and generally coincides with the RPI minimum from MD07-3076Q (Figs. 6 and 9e).

The Iceland Basin excursion coincides with the later part of the MIS 6e/7a boundary (terminology of Railsback et al., 2015) in the Antarctic δD record (Fig. 9b,d) which yields a mid-point excursion age of 188.5 ka and duration of ~3.5 kyr. Both age and duration estimates depend on the criteria for defining the excursion. Here we use component magnetization inclination values >-40° to define the directional excursion. For the Iceland Basin excursion recorded in the North Atlantic Ocean, estimated mid-point ages are in the 189-190 ka range (Channell, 2014).

The two RPI proxies (slopes of NRM/ARM and NRM/IRM) are inconsistent in the 0-20 ka interval, which also corresponds to a notable departure from the RPI reference templates (SAPIS and PISO) (Fig. 6). This interval, in turn, corresponds to high concentrations of biogenic magnetite. We conclude that the presence of biogenic magnetite results in over-normalization when using NRM/ARM as the RPI proxy and under-normalization when using NRM/IRM (see Ouyang et al., 2014). In addition, the calibrated RPI templates (e.g. SAPIS and PISO) are poorly defined in the last ~30 kyr, due to their reliance on ODP and MD cores which are characterized by poor core-quality in the uppermost few meters. The fit of MD07-3076Q/3077 RPI proxies to PISO/SAPIS is also poor in the 60-80 ka interval, and RPI minima at ~115 ka and ~190 ka are offset (Fig. 6), possibly implying poor definition of the RPI templates. The large (25 x 25 cm) square-section piston core (MD07-3076Q) is likely to be less affected by stretching artifacts compared to ODP and MD cores.

Below the interval of high biogenic magnetite concentration (0-20 ka), a plausible fit of RPI proxies to the reference templates can be made down to depths corresponding to ~250 ka (Fig. 6).
6). Prior to 250 ka, below ~32 mbsf, RPI proxies are muted relative to the templates, NRM intensities are decreased (Fig. 3), and the MDF of NRM increases. These changes are attributed to progressive magnetite dissolution as a result of microbial sulfate reduction, formation of pore-water sulfide, and reduction of magnetite to form iron sulfides, a process that is ubiquitous in pelagic sediments other than highly oxidized facies such as Pacific red clays. The enhanced reactivity of fine grains results in magnetite grain-size coarsening down-core as dissolution proceeds. The presence of ultra-fine (biogenic) magnetite, from TEM observations (Fig. 8) and rock magnetic data (Fig. 7), imply high concentrations of biogenic magnetite in the uppermost sediments.

353

## 9. Conclusions

The Laschamp and Iceland Basin excursions are documented in cores MD07-3076Q and MD07-3077, respectively, by discrete samples and u-channel samples (Fig. 9). Recording of the Laschamp and Iceland Basin excursions is facilitated by sedimentation rates of ~20 cm/kyr and ~10 cm/kyr in the two excursion intervals, respectively (Fig. 2a). The millennial-scale duration of the Laschamp and Iceland Basin directional excursions (~1 kyr and ~3.5 kyr, respectively) results in rare recordings of excursions in sedimentary sequences with normal pelagic sedimentation rates (<10 cm/kyr). Sedimentary records of magnetic excursions are often compromised by bioturbation and non-instantaneous magnetic remanence acquisition, the inherent brevity of magnetic excursions, and by down-core magnetite dissolution that preferentially affects fine-grained biogenic magnetite.

The documentation and timing of the Laschamp and Iceland Basin excursions in the South Atlantic Ocean augments the few other recordings of these excursions in the southern
hemisphere (cited above). The age models for cores MD07-3076Q and MD07-3077 were built through radiocarbon and sea-surface temperature proxies tied to Antarctic ice core records, and are supported by the resulting coincidence of the marine Laschamp excursion record with cosmogenic $^{10}$Be and nitrate fluxes in the EDC Antarctic ice core (Fig. 9). Excursion mid-point ages and durations determined from the age models are 40.9 ka and ~1 kyr for the Laschamp excursion, and 188 ka and ~3.5 kyr for the Iceland Basin excursion, respectively, using inclination values >-40° to define the stratigraphic intervals over which the excursions are recorded (Fig. 9). We observe no discrepancies in the ages of the Laschamp or Iceland Basin excursions between the North and South Atlantic Oceans as has been mooted for both the M-B reversal and the Laschamp excursion (Leonhardt and Fabian, 2007; Leonhardt et al., 2009).

377

378

Acknowledgements

We thank Kainian Huang for laboratory assistance, and Andrew Roberts for a thorough review of the manuscript. This research was supported by US National Science Foundation grants OCE-0850413 and EAR-1014506 (to JC), NERC grant NE/J010545/1 (to LCS), Gates Cambridge Scholarship (to JG), and AXA Research Fund Post-doctoral Fellowship (to NVR). CW acknowledges support from European Research Council grant ACCLIMATE/no 339108. This is LSCE contribution 5666.
385 References

386 Banerjee, S.K., Mellema, J.P., 1974. A new method for the determination of paleointensity from
387 the ARM properties of rocks. Earth Planet. Sci. Lett., 23, 177-184.

388 Bazin, L., Landais, A., Lemieux-Dudon, B., Toyé Mahamadou Kele, H., Veres, D., Parrenin, F.,
389 Martinerie, P., Ritz, C., Capron, E., Lipenkov, V., Loutre, M.F., Raynaud, D., Vinther,
390 B.M., Svensson, A., Rasmussen, S.O., Severi, M., Blunier, T., Leuenberger, M., Fischer, H.,
391 Masson-Delmotte, V., Chappellaz, J., Wolff, E., 2013. An optimized multi-proxy, multi-site
392 Antarctic ice and gas orbital chronology (AICC2012): 120 - 800 ka. Clim. Past 9, 1715-
393 1731.

394 Blunier, T., Brook, E., 2001. Timing of millennial-scale climate change in Antarctica and
395 Greenland during the last glacial period. Science, 291, 109-112.

396 Carter-Stiglitz, B., Moskowitz, B., Jackson, M., 2001. Unmixing magnetic assemblages and the
397 magnetic behavior of bimodal mixtures. J. Geophys. Res., 106, 26,397-26, 411.

398 Chang, L., Roberts, A.P., Williams, D., Fitz Gerald, J.D., Larrasoana, J.C., Jovane L.,
399 Muxworthy, A.R., 2012. Giant magnetofossils and hyperthermal events. Earth Planet. Sci.
400 Letters, 351-352, 258-269.

401 Channell, J.E.T., 1999. Geomagnetic paleointensity and directional secular variation at Ocean
402 Drilling Program (ODP) Site 984 (Bjorn Drift) since 500 ka: comparisons with ODP Site
403 983 (Gardar Drift). J. Geophys. Res., 104, 22,937-22,951.

404 Channell, J.E.T., 2006. Late Brunhes polarity excursions (Mono Lake, Laschamp, Iceland Basin
405 and Pringle Falls) recorded at ODP Site 919 (Irminger Basin). Earth Planet. Sci. Lett., 244,
406 378-393.
407 Channell, J.E.T., 2014. The Iceland Basin excursion: age, duration, and excursion field
408 geometry. Geochem. Geophys. Geosyst., 15, 4920-4935, doi:10.1002/2014GC005564.

409 Channell, J.E.T., Hodell D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and δ¹⁸O at ODP Site 983 (Gardar Drift, North Atlantic) since 350 ka. Earth Planet. Sci. Lett., 153, 103-118.

412 Channell, J.E.T., Stoner, J.S., Hodell, D.A., Charles, C.D., 2000. Geomagnetic paleointensity for the last 100 kyr from the sub-antarctic South Atlantic: a tool for inter-hemispheric correlation. Earth Planet. Sci. Lett., 175, 145-160.

415 Channell, J.E.T., Mazaud, A., Sullivan, P., Turner, S., and Raymo, M.E., 2002. Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin). J. Geophys. Res., 107, 2114, doi:10.1029/2001JB000491.

418 Channell, J.E.T., Xuan, C. and Hodell, D.A., 2009. Stacking paleointensity and oxygen isotope data for the last 1.5 Myrs (PISO-1500). Earth Planet. Sci. Lett., 283, 14-23.

420 Channell, J.E.T., Hodell, D.A., Singer, B.S., Xuan, C., 2010. Reconciling astrochronological and ⁴⁰Ar/³⁹Ar ages for the Matuyama-Brunhes boundary and late Matuyama Chron. Geochem. Geophys. Geosyst., 11, Q0AA12, doi:10.1029/2010GC003203.

423 Channell, J.E.T., Hodell, D.A., Curtis, J.H., 2012. ODP Site 1063 (Bermuda Rise) revisited: oxygen isotopes, excursions and paleointensity in the Brunhes Chron. Geochem. Geophys. Geosyst., 13, Q02001, doi:10.1029/2011GC003897.

426 Channell, J.E.T., Hodell, D.A., Margari, V., Skinner, L.C., Tzedakis, P.C., Kesler, M.S., 2013. Biogenic magnetite, detrital hematite, and relative paleointensity in sediments from the Southwest Iberian Margin, Earth Planet. Sci. Lett., 376, 99-109.
429 Charles, C.D., Lynch-Stieglitz, J., Ninnemann, U.S., Fairbanks R.G., 1996. Climate connections between the hemispheres revealed by deep sea sediment core/ice core correlations. Earth Planet. Sci. Lett., 142, 19-27.

432 Collins, L.G., Hounslow, M.W., Allen, C.S., Hodgson, D.A., Pike, J., Karloukovski, V.V., 2012. Palaeomagnetic and biostratigraphic dating of marine sediments from the Scotia Sea, Antarctica: First identification of the Laschamp excursion in the Southern Ocean. Quart. Geochron., 7, 67-75.

436 Day, R., Fuller, M., Schmidt, V.A., 1977. Hysteresis properties of titanomagnetites: grain-size and compositional dependence. Phys. Earth Planet. Inter., 13, 260-267.

438 Dunlop, D.J., 2002. Theory and application of the Day plot ($M_{rs}/M_s$ versus $H_{cr}/H_c$) 1. Theoretical curves and tests using titanomagnetite data. J. Geophys. Res., 107, B3, 2056, doi:10.1029/2001JB000486.

441 Dunlop, D.J., Carter-Stiglitz, B., Day plots of mixtures of superparamagnetic, single domain, pseudosingle domain, and multidomain magnetites. J. Geophys. Res., 111, B12S09, doi: 10.1029/2006JB004499.

444 Egli, R., Chen, A.P., Winklhofer, M., Kodama K.P., Horng, C-S., 2010. Detection of non-interacting single domain particles using first-order reversal curve diagrams. Geochem. Geophys. Geosyst., 11, Q01Z11, doi:10.1029/2009GC002916.

447 Evans, H.F., Channell, J.E.T., Stoner, J.S., Hillaire-Marcel, C., Wright, J.D., Neitzke, L.C., Mountain, G.S., 2007. Paleointensity-assisted chronostratigraphy of detrital layers on the Eirik Drift (North Atlantic) since marine isotope stage 11. Geochem. Geophys. Geosyst., 8, Q11007, doi: 10.1029/2007GC001720.
451 Gottschalk, J., Skinner, L.C., Waelbroeck, C., 2015a. Contribution of seasonal sub-Antarctic
452 surface water variability to millennial-scale changes in atmospheric CO₂ over the last
453 deglaciation and Marine Isotope Stage 3. Earth Planet. Sci. Letters, 411, 87-99.

454 Gottschalk, J., Skinner, L.C., Misra, S., Waelbroeck, C., Menviel, L., Timmermann, A., 2015b.
455 Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard-
456 Oeschger events, Nature Geosci., 8, 950-955.

457 Guyodo, Y., Channell, J.E.T., Thomas, R., 2002. Deconvolution of u-channel paleomagnetic
458 data near geomagnetic reversals and short events. Geophys. Res. Letters, 29, 1845,
459 doi:10.1029/2002GL014963.

460 Harrison, R.J., Feinberg, J.M., 2008. FORCinel: an improved algorithm for calculating first-
461 order reversal curve distributions using locally weighted regression smoothing. Geochem.,
462 Geophys., Geosyst., 9, Q05016, doi:10.1029/2008GC001987.

463 Hodell, D. A., Charles, C.D., Sierro, F., 2001. Late Pleistocene evolution of the ocean'scarbonate
464 system, Earth Planet. Sci. Lett., 192, 109-124.

465 Jouzel, J., Masson-Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., Minster,
466 B., Nouet, J., Barnola, J.M., Chappellaz, J., Fischer, H., Gallet, J.C., Johnsen, S.,
467 Leuenberger, M., Loulergue, L., Luethi, D., Oerter, H., Parrenin, F., Raisbeck, G., Raynaud,
468 D., Schilt, A., Schwander, J., Selmo, E., Souchez, R., Spahni, R., Stauffer, B., Steffensen,
469 J.P., Stenni, B., Stocker, T.F., Tison, J.L., Werner, M., Wolff, E.W., 2007. Orbital and
470 millennial Antarctic climate variability over the past 800,000 years. Science, 317, 793-796.

471 King, J.W., Banerjee, S.K., Marvin, J., 1983. A new rock-magnetic approach to selecting
472 sediments for geomagnetic paleointensity studies: application to paleointensity for the last
473 4000 years. J. Geophys. Res. 88, 5911-5921.
474 Kirschvink, J.L., 1980. The least squares lines and plane analysis of paleomagnetic data.

475 Geophys. J.R. Astron. Soc. 62, 699-718.

476 Kopp, R.E., Kirschvink, J.L., 2008. The identification and biogeochemical interpretation of fossil

477 magnetotactic bacteria. Earth Sci. Rev., 86, 42-61.

478 Laj, C., Kissel, C., Mazaud, A., Channell, J.E.T., Beer, J., 2000. North Atlantic paleointensity

479 stack since 75 ka (NAPIS-75) and the duration of the Laschamp event. Phil. Trans. R. Soc.

480 Lond., 358, 1009-1025.

481 Laj, C., Kissel, C., Roberts, A.P., 2006. Geomagnetic field behavior during the Icelandic Basin

482 and Laschamp geomagnetic excursions: a simple transitional field geometry? Geochem.

483 Geophys. Geosyst., 7, Q03004, doi:10,1029/2005GC001122.

484 Laj, C., Guillou, H., Kissel, C., 2014. Dynamics of the Earth's magnetic field in the 10-75 kyr

485 period comprising the Laschamp and Mono Lake excursions: new results from the French

486 Chaine des Puys in a global perspective. Earth Planet. Sci. Lett., 387, 184-197.

487 Lamy, F., Kaiser, J., Ninnemann, U., Hebbeln, D., Arz H.W., Stoner, J., 2004. Antarctic timing

488 of surface water changes off Chile and Patagonian Ice Sheet response. Science, 304, 1959-

489 1962.

490 Lascu, I., Feinberg, J.M., Dorale, J.A., Cheng, H., Edwards, R.L., 2016. Age of the Laschamp

491 excursion determined by U-Th dating of a speolthem record from North America. Geology,

492 44, 139-142, doi:10.1130/G37490.1.

493 Leonhardt, R., Fabian, K., 2007. Paleomagnetic reconstructon of the global geomagnetic field

494 evolution during the Matuyama/Brunhes transition: iterative Bayesian inversion and

495 independent verification. Earth Planet. Sci. Lett., 253, 172-195.

496 Leonhardt, R., Fabian, K., Winklhofer, M., Ferk, A., Laj C., Kissel, C., 2009. Geomagnetic field
497 evolution during the Laschamp excursion. Earth Planet. Sci. Lett., 278, 87-95.

498 Levi, S., Banerjee, S.K., 1976. On the possibility of obtaining relative paleointensities from lake

499 sediments. Earth Planet. Sci. Lett., 29, 219-226.

500 Lund, S.P., Schwartz, M., Keigwin, L., Johnson, T., 2005. Deep-sea sediment records of the

501 Laschamp geomagnetic field excursion (<41,000 calendar years before present). J. Geophys.

502 Res., 110, Q12006, doi:10.1029, 2005GC001036.

503 Lund, S., Stoner, J.S., Channell, J.E.T., Acton, G., 2006a. A summary of Brunhes paleomagnetic

504 field variability recorded in Ocean Drilling Program cores. Phys. Earth Planet. Inter., 156,

505 194-204.

506 Lund, S.P., Stoner, J.S., Lamy, F., 2006b. Late Quaternary paleomagnetic secular variation and

507 chronostratigraphy from ODP Sites 1233 and 1234. In: Tiedemann, R., Mix, A.C., Richter,

508 C., and Ruddiman, W.F. (Eds.), Proc. ODP Sci. Results, 202: College Station, TX (Ocean

509 Drilling Program), 1-22. doi:10.2973/odp.proc.sr.202.208.2006.

510 Mazaud, A., Sicre, M.A., Ezat, U., Pichon, J.J., Duprat, J., Laj, C., Kissel, C., Beaufort, L.,

511 Michel, E., Turon, J.L., 2002. Geomagnetic-assisted stratigraphy and sea surface

512 temperature changes in core MD94-103 (Southern Indian Ocean): possible implications for

513 North-South climatic relationships around H4. Earth Planet. Sci. Lett., 201, 159-170.

514 Mortyn, P.G., Charles, C.D., Ninnemann, U.S., Ludwig K., Hodell, D.A., 2003. Deep sea

515 sedimentary analogs for the Vostok ice core. Geochem., Geophys., Geosyst., 4, 8405,

516 doi:10.1029/2002GC000475.

517 Muxworthy, A.R., Roberts, A.P., 2007. First-order reversal curve (FORC) diagrams. In

518 Encyclopedia of Geomagnetism and Paleomagnetism. Gubbins, D. and E. Herrero-Bervera

519 (eds.), 266-272, Springer, Dordrecht, Netherlands.
520 Nowaczyk, N.R., Frank, U., Kind, J., Arz, H.W., 2013. A high-resolution paleointensity stack of the past 14 to 68 ka from Black Sea sediments. Earth Planet. Sci. Lett., 384, 1-16, 2013.

522 Oda, H., Nakamura, K., Ikehara, K., Nakano, T., Nishimura, M., Khlystov, O., 2002.

523 Paleomagnetic record from Academician Ridge, Lake Baikal: a reversal excursion at the base of marine oxygen isotope stage 6. Earth Planet. Sci. Lett., 202, 117-132.

525 Ouyang, T., Heslop, D., Roberts, A.P., Tian, C., Zhu, Z., Qiu, Y., Peng, X., 2014. Variable remanence acquisition efficiency in sediments containing biogenic and detrital magnetites: implications for relative paleointensity signal recording. Geochem. Geophys. Geosyst., 15, 2780-2796, doi:10.1002/2014GC005301.

529 Pike, C.R., Roberts A.P., Verosub, K.L., 1999. Characterizing interactions in fine magnetic particle systems using first order reversal curves. J. Appl. Phys., 85, 6660-6667.

531 Railsback, L.B., Gibbard, P.L., Head, M.J., Voarintsoa N.G.R., Toucanne, S., 2015. An optimized scheme of lettered marine isotope substages for the last 1.0 million years, and the climatostratigraphic nature of isotope stages and substages. Quat. Sci. Rev., 111, 94-106.

534 Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in $^{40}$Ar/$^{39}$Ar dating. Chem. Geol., 145, 117-152.

537 Renne, P.R., Mundil, R., Balco, G., Min K., Ludwig, K.R., 2010. Joint determination of $^{40}$K decay constants and $^{40}$Ar/$^{40}$K for the Fish Canyon sanidine standard, and improved accuracy for $^{40}$Ar/$^{39}$Ar geochronology. Geochim. Cosmochim. Acta, 74, 5349-5367.

540 Roberts, A.P., Lehman, B., Weeks, R.J., Verosub K.L., Laj, C., 1997. Relative paleointensity of the geomagnetic field over the last 200,000 years from ODP Sites 883 and 884, North Pacific Ocean. Earth Planet. Sci. Lett., 152, 11-23.
543 Roberts, A.P., Pike, C.R., Verosub, K.L., 2000. First-order reversal curve diagrams: a new tool
544 for characterizing the magnetic properties of natural samples. J. Geophys. Res., 105, 28,461-
545 28,475.

546 Roberts, A.P., Florindo, F., Villa, G., Chang, L., Jovane, L., Bohaty, S.M., Larrasoana, J.C.,
547 Heslop, D., Fitz Gerald, J.D., 2011. Magnetotactic bacterial abundance in pelagic marine
548 environments is limited by organic carbon flux and availability of dissolved iron. Earth
549 Planet. Sci. Lett., 310, 441-452.

550 Roberts, A.P., Chang, L., Heslop, D., Florindo, F., Larrasoana, J.C., 2012. Searching for single
551 domain magnetite in the “pseudo-single-domain” sedimentary haystack: implications of
552 biogenic magnetite preservation for sediment magnetism and relative paleointensity
553 determinations. J. Geophys. Res., 117, B08104, doi:10.1029/2012JB009412.

554 Skinner, L.C., McCave, I.N., 2003. Analysis and modeling of gravity- and piston coring based
555 on soil mechanics. Mar. Geol., 199, 181-209.

556 Skinner, L.C., Fallon, S., Waelbroeck, C., Michel E., Barker, S., 2010. Ventilation of the Deep
557 Southern Ocean and deglacial CO₂ rise. Science, 328, 1147-1151.

558 Stoner, J.S., Laj, C., Channell, J.E.T., Kissel, C., 2002. South Atlantic (SAPIS) and North
559 Atlantic (NAPIS) geomagnetic paleointensity stacks (0-80 ka): implications for inter-
560 hemispheric correlation. Quat. Sci. Rev., 21, 1141-1151.

561 Stoner, J.S., Channell, J.E.T., Hodell, D.A., Charles, C., 2003. A 580 kyr paleomagnetic record
562 from the sub-Antarctic South Atlantic (ODP Site 1089). J. Geophys. Res., 108, 2244,
563 doi:10.1029/2001JB001390.

564 Stoner, J.S., Lund, S., Channell, J.E.T., Mix, A.C., Davies, M.H., Lamy, F., 2008. High
565 sedimentation rate paleomagnetic records for the last 70 kyrs from the Chilean Margin (ODP
566 Sites 1233, 1234, 1235). American Geophysical Union, Fall Meeting, Abstract GP14A-02,
567 San Francisco.

568 Svensson, A., Andersen, K.K., Bigler, M., Clausen, H.B., Dahl-Jensen, D., Davies, S.M.,
569 Johnsen, S.J., Muscheler, R., Rasmussen, S.O., Rothlisberger, R., Steffensen, J.P., Vinther,
570 B.M., 2006. The Greenland Ice Core Chronology 2005, 15-42 ka. Part 2: comparison to
571 other records. Quat. Sci. Rev., 25, 3258-3267.

572 Svensson, A., Andersen, K.K., Bigler, M., Clausen, H.B., Dahl-Jensen, D., Davies, S.M.,
573 Johnsen, S.J., Muscheler, R., Parrenin, F., Rasmussen, S.O., Rothlisberger, R., Seierstad, I.,
574 Steffensen, J.P., Vinther, B.M., 2008. A 60,000 year Greenland stratigraphic ice core
575 chronology. Clim. Past, 4, 47-57.

576 Széréméta, N., Bassinot, F., Balut, Y., Labeyrie, L., Pagel, M., 2004. Oversampling of
577 sedimentary series collected by giant piston corer: evidence and corrections based on 3.5-
578 kHz chirp profiles. Paleoceanography, 19, PA1005, doi:10.1029/2002PA000795.

579 Tauxe, L., 1993. Sedimentary records of relative paleointensity of the geomagnetic field: theory
580 and practice. Rev. Geophys., 31, 319-354.

581 Thomas, R., Guyodo, Y., Channell, J.E.T., 2003. U-channel track for susceptibility
582 measurements. Geochem., Geophys. Geosyst., 1050, doi:10.1029/2002GC000454.

583 Traversi, R., Becagli, S., Poluianov, S., Severi, M., Solanki, S.K., Usoskin I.G., Udisti, R., 2016.
584 The Laschamp geomagnetic excursion featured in nitrate record from EPICA-Dome C ice
585 core. Sci. Rep., 6, 20235; doi: 10-1038/srep20235.

586 Vázquez Riveiros, N., Waelbroeck, C., Skinner, L., Roche, D.M., Duplessy J-C., Michel, E.,
587 2010. Response of South Atlantic deep waters to deglacial warming during Termination V
588 and I. Earth Planet. Sci. Letters, 298, 323-333.
589 Vázquez Riveiros, N., Waelbroeck, C., Skinner, L., Duplessy, J-C., McManus, J.F., Kandiano, K.S., Bauch, H.A., 2013. The “MIS 11 paradox” and ocean circulation: role of millennial scale events. Earth Planet. Sci. Lett., 371-372, 258-268.

592 Veres, D., Bazin, L., Landais, A., Kele, H.T.M., Lemieux-Dudon, B., Parrenin, F., Martinerie, P., Blayo, E., Blunier, T., Capron, E., Chappellaz, J., Rasmussen, S.O., Severi, M., Svensson, A., Vinther, B.M., Wolff, E., 2013. The Antarctic ice core chronology (AICC2012): an optimized multi-parameter and multi-site dating approach for the last 120 thousand years. Clim. Past 9, 1733-1748.

597 Voelker, A., Sarnthein, M. Grootes, P.M., Erlenkeuser, H., Laj, C., Mazaud, A., Nadeau, M.J., Schleicher, M., 1998. Correlation of marine $^{14}$C ages from the Nordic sea with GISP2 isotope record: implication for $^{14}$C calibration beyond 25 ka BP. Radiocarbon, 40, 517-534.

600 Weeks, R., Laj, C., Endignoux, L., Fuller, M., Roberts, A., Manganne, R., Blanchard, E., Goree, W., 1993. Improvements in long-core measurement techniques: applications in palaeomagnetism and palaeoceanography. Geophys. J. Int., 114, 651-662.

603 Xuan, C., Channell, J.E.T., 2009. UPmag: MATLAB software for viewing and processing u-channel or other pass-through paleomagnetic data. Geochem. Geophys. Geosyst., 10, Q10Y07, doi:1029/2009GC002584.

606 Yamazaki, T., 2012. Paleoposition of the intertropical convergence zone in the eastern Pacific inferred from glacial-interglacial changes in terregenous and biogenic magnetic mineral fractions. Geology, 40, 151-154.

609 Yamazaki, T., Ioka, N., 1994. Long-term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial Pacific, Earth Planet. Sci. Lett., 128, 527-544.
612 Yiou, F., Raisbeck, G.M., Baumgartner, S., Beer, J., Hammer, C., Johnsen, S., Jouzel, J., Kubik,

613 P.W., Lestringuez, J., Stiévenard, M., Suter, M., Yiou, P., 1997. Beryllium 10 in Greenland

614 Ice Core Project ice core at Summit, Greenland. J. Geophys. Res., 102, 26783-26794.

615
616 **Figure captions**

617 Fig. 1. Map with locations of cores MD07-3076Q and MD07-3077, and ODP Site 1089
618 (from GeoMapApp™).

619 Fig. 2. Age models for the studied sediment cores. (a) Inferred sedimentation rates for core
620 MD07-3076Q (red) and MD07-3077 (blue) with age tie-points (blue/red points) listed in Tables
621 1 and 2. (b) δD from the EPICA Dome C (EDC) ice core (Jouzel et al., 2007) placed on the
622 AICC2012 chronology (Bazin et al., 2013; Veres et al., 2013). (c) Percentage of *G. bulloides* for
623 cores MD07-3076Q (red) and MD07-3077 (blue). (d) Percentage *N. pachyderma* (sin.) for cores
624 MD07-3076Q (red) and MD07-3077 (blue). (e) Blue reflectance for cores MD07-3076Q (red)
625 and MD07-3077 (blue). At base: expanded versions of (b), (c) and (d) for core MD07-3076Q in
626 the 24-68 ka interval (left), and for core MD07-3077 in the 145-194 ka interval (right). For
627 percentages of *G. bulloides* and *N. pachyderma*, the data points, smoothed lines, and standard
628 deviations are shown.

629 Fig. 3. Component magnetization directions with stratigraphic position of the Laschamp and
630 Iceland basin excursions in cores MD07-3076Q (red) and MD07-3077 (blue). Component
631 declination, inclination and maximum angular deviation (MAD) values computed for the 20-80
632 mT demagnetization interval from u-channel samples plotted versus depth (meters below
633 seafloor, mbsf). Natural remanent magnetization (NRM) intensities after 20 mT peak AF
634 demagnetization decrease below ~32 mbsf, attributed to diagenetic magnetite dissolution.

635 Fig. 4. U-channel and discrete sample paleomagnetic data for the Laschamp excursion (left:
636 core MD07-3076Q) and Iceland Basin excursion (right: core MD07-3077) versus depth (meters
637 below seafloor, mbsf). Component inclinations (red symbols with line for u-channel data, orange
638 triangles for discrete samples), component declinations (blue circles for u-channel data, light
639 blue triangles for discrete samples) and maximum angular deviation (MAD) values for u-
640 channels (green dots) and discrete samples (black triangles). Shaded intervals indicate intervals
641 yielding clearly excursional magnetization directions, defined here by inclination values >-40°.

642 Fig. 5. Orthogonal projections of alternating field demagnetization data for (a) discrete
643 samples in the vicinity of the Laschamp excursion (core MD07-3076Q), (b) discrete samples in
644 the vicinity of the Iceland Basin excursion (core MD07-3077), (c) u-channel samples that record
645 the Laschamp excursion in core MD07-3076Q and the Iceland Basin excursion in core MD07-
646 3077. Red lines/symbols represent projections onto the vertical plane. Blue lines/symbols
647 represent projections onto horizontal plane. Depths in the cores and ages associated with each
648 orthogonal projection are indicated. Natural remanent magnetization (NRM) measurements prior
649 to demagnetization were followed by demagnetization at peak fields of 10-50 mT in 2.5 mT
650 steps, and between 50 and 90 mT in 5 mT steps. All projections have West and Up towards top
651 of page. Magnetization intensities x10⁻² A/m.

652 Fig. 6. PISO relative paleointensity (RPI) stack (black) (Channell et al., 2009) with SAPIS
653 RPI stack (green) (Stoner et al., 2002). Core MD07-3076Q and MD07-3077 RPI proxies:
654 NRM/ARM (light blue/blue) and NRM/IRM₀.₃T (orange/red) placed on the adopted age model.

655 Fig. 7. (a) Plot of anhysteretic susceptibility (κARM) versus susceptibility (κ) for core MD07-
656 3076Q (blue dots, purple triangles are from the Holocene interval) and MD07-3077 (red dots)
657 with calibration of magnetite grain size from King et al. (1983). (b) Hysteresis ratio plot after
658 Day et al. (1977) for samples from core MD07-3076Q (black squares) and core MD07-3077 (red
circles), which lie along a magnetite grain-size mixing line (blue triangles) in the pseudo-single domain (PSD) field, between the single domain (SD) and multidomain (MD) fields. Green squares: hysteresis ratios from crushed, sized (unannealed) natural titanomagnetite (Dunlop, 2002). (c) First-order reversal curve (FORC) diagram for Holocene sediments at 0.5 mbsf in core MD07-3076Q (10.07 ka). (d) FORC diagram from the last glacial maximum at 1.31 m in core MD07-3076Q (18.84 ka). FORC diagrams were processed using a smoothing factor of 6.

Fig. 8. Transmission electron microscope (TEM) images of magnetic mineral extracts. (a) Typical biogenic magnetite grains from a depth of 0.86-0.90 mbsf in core MD07-3076Q. (b) Typical biogenic magnetite grains with a detrital magnetite grain at the top right from the same magnetic extract (0.86-0.90 mbsf) with position (red line) of an energy dispersive x-ray spectroscopy (EDS) line scan, with counts from the line scan (right). Presence or absence of Ti indicates detrital or biogenic magnetite, respectively. (c) Detrital magnetite grain from a depth of 2.20-2.30 mbsf in core MD07-3076Q with the position (red dot) of EDS analysis (right). White bars provide scales for each micrograph.

Fig. 9. Laschamp (left) and Iceland Basin excursion (right) in cores MD07-3076Q and MD07-3077, respectively, versus age (ka). (a and b) Percentage of *N. pachyderma* (sin.) (blue) and *G. bulloides* (red) compared with δD (black) from the EPICA (EDC) ice core (Jouzel et al., 2007) placed on the AICC2012 chronology (Bazin et al., 2013; Veres et al., 2013). Antarctic Isotopic Maxima (AIM 8-11, EPICA Community members, 2006) and Greenland Stadials (GS) and Interstadials (GI) correlated after Veres et al. (2013). (c and d) Component inclinations (red dots with line for u-channel data, orange triangles for discrete samples), component declinations (blue circles for u-channel data, light blue triangles for discrete samples) plotted versus age.
681 Component declinations/inclinations computed for a uniform 20-80 mT peak AF

682 demagnetization interval for both u-channel and discrete sample data. Shaded intervals

683 demarcate clear excursional directions, defined here by inclination values >-40°. (e) EPICA

684 cosmogenic fluxes for 38.5-44.0 ka (Traversi et al., 2016) for ¹⁰Be (orange) with a 10 point

685 running mean (red), and nitrate (light green) with a 10 point running mean (dark green),

686 compared with RPI proxies from core MD07-3076Q (from Fig. 6 with NRM/ARM slope in dark

687 blue and NRM/IRM slope in light blue).
<table><caption>Table 1. Age model for Core MD07-3076Q</caption><thead><tr><th>Depth (cm)</th><th>Age (ka)</th><th>Ref./Basis</th><th>Depth (cm)</th><th>Age (ka)</th><th>Ref./Basis</th></tr></thead><tbody><tr><td>1.5000</td><td>1.2030</td><td>1/radiocarb.</td><td>113.50</td><td>17.113</td><td>1/radiocarb.</td></tr><tr><td>5.5000</td><td>1.6250</td><td>1/radiocarb.</td><td>117.50</td><td>17.578</td><td>1/radiocarb.</td></tr><tr><td>9.5000</td><td>2.2400</td><td>1/radiocarb.</td><td>119.50</td><td>17.802</td><td>1/radiocarb.</td></tr><tr><td>15.500</td><td>3.2970</td><td>1/radiocarb.</td><td>121.50</td><td>18.006</td><td>1/radiocarb.</td></tr><tr><td>21.500</td><td>4.7340</td><td>1/radiocarb.</td><td>125.50</td><td>18.381</td><td>1/radiocarb.</td></tr><tr><td>25.500</td><td>5.6580</td><td>1/radiocarb.</td><td>133.50</td><td>19.042</td><td>1/radiocarb.</td></tr><tr><td>31.500</td><td>6.5430</td><td>1/radiocarb.</td><td>137.50</td><td>19.403</td><td>1/radiocarb.</td></tr><tr><td>35.500</td><td>7.0180</td><td>1/radiocarb.</td><td>145.50</td><td>20.512</td><td>1/radiocarb.</td></tr><tr><td>41.500</td><td>8.4550</td><td>1/radiocarb.</td><td>161.50</td><td>22.881</td><td>1/radiocarb.</td></tr><tr><td>45.500</td><td>9.3530</td><td>1/radiocarb.</td><td>173.50</td><td>24.743</td><td>1/radiocarb.</td></tr><tr><td>49.500</td><td>10.003</td><td>1/radiocarb.</td><td>187.50</td><td>26.234</td><td>1/radiocarb.</td></tr><tr><td>53.500</td><td>10.569</td><td>1/radiocarb.</td><td>197.50</td><td>27.380</td><td>1/radiocarb.</td></tr><tr><td>55.500</td><td>10.847</td><td>1/radiocarb.</td><td>237.00</td><td>30.062</td><td>% G. bulloides</td></tr><tr><td>59.500</td><td>11.277</td><td>1/radiocarb.</td><td>290.60</td><td>33.655</td><td>% G. bulloides</td></tr><tr><td>61.500</td><td>11.538</td><td>1/radiocarb.</td><td>325.60</td><td>35.848</td><td>% G. bulloides</td></tr><tr><td>63.500</td><td>11.842</td><td>1/radiocarb.</td><td>368.40</td><td>38.264</td><td>% G. bulloides</td></tr><tr><td>67.500</td><td>12.536</td><td>1/radiocarb.</td><td>427.30</td><td>41.264</td><td>% G. bulloides</td></tr><tr><td>73.500</td><td>13.043</td><td>1/radiocarb.</td><td>459.80</td><td>43.092</td><td>% G. bulloides</td></tr><tr><td>77.500</td><td>13.405</td><td>1/radiocarb.</td><td>491.60</td><td>45.928</td><td>% G. bulloides</td></tr><tr><td>85.500</td><td>14.362</td><td>1/radiocarb.</td><td>595.90</td><td>53.903</td><td>% G. bulloides</td></tr><tr><td>89.500</td><td>14.670</td><td>1/radiocarb.</td><td>687.30</td><td>59.576</td><td>% G. bulloides</td></tr><tr><td>91.500</td><td>14.792</td><td>1/radiocarb.</td><td>760.40</td><td>66.111</td><td>% G. bulloides</td></tr><tr><td>95.500</td><td>14.938</td><td>1/radiocarb.</td><td>906.70</td><td>84.941</td><td>Refl./3077</td></tr><tr><td>99.500</td><td>15.205</td><td>1/radiocarb.</td><td>991.70</td><td>95.571</td><td>Refl./3077</td></tr><tr><td>109.50</td><td>16.590</td><td>1/radiocarb.</td><td>1089.8</td><td>107.08</td><td>Refl./3077</td></tr></tbody></table>

Ref 1: Skinner et al. (2010)
<table><caption>Table 2. Age model for Core MD07-3077</caption><thead><tr><th>Depth (cm)</th><th>Corrected depth (cm)</th><th>Age (ka) AICC2012</th><th>Basis</th></tr></thead><tbody><tr><td>0.0000</td><td>0.0000</td><td>1.0450</td><td>Refl. from MD07-3076Q</td></tr><tr><td>197.10</td><td>197.10</td><td>18.682</td><td>Refl. from MD07-3076Q</td></tr><tr><td>292.00</td><td>292.00</td><td>27.323</td><td>Refl. from MD07-3076Q</td></tr><tr><td>647.20</td><td>647.20</td><td>36.492</td><td>Refl. from MD07-3076Q</td></tr><tr><td>768.00</td><td>768.00</td><td>39.975</td><td>Refl. from MD07-3076Q</td></tr><tr><td>901.20</td><td>901.20</td><td>43.850</td><td>Refl. from MD07-3076Q</td></tr><tr><td>1023.2</td><td>1023.2</td><td>48.428</td><td>Refl. from MD07-3076Q</td></tr><tr><td>1397.0</td><td>1397.0</td><td>64.806</td><td>Refl. from MD07-3076Q</td></tr><tr><td>1578.1</td><td>1578.1</td><td>77.122</td><td>Planktic δ<sup>18</sup>O</td></tr><tr><td>1700.1</td><td>1700.1</td><td>88.143</td><td>Planktic δ<sup>18</sup>O</td></tr><tr><td>1932.0</td><td>1932.0</td><td>108.39</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2030.6</td><td>1989.6</td><td>119.57</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2068.5</td><td>2027.5</td><td>126.03</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2136.3</td><td>2095.3</td><td>131.80</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2164.9</td><td>2123.9</td><td>136.15</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2305.8</td><td>2264.8</td><td>150.58</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2345.2</td><td>2304.2</td><td>154.51</td><td>% G. bulloides</td></tr><tr><td>2379.6</td><td>2338.6</td><td>156.78</td><td>% G. bulloides</td></tr><tr><td>2399.8</td><td>2358.8</td><td>160.26</td><td>% G. bulloides</td></tr><tr><td>2441.8</td><td>2400.8</td><td>163.67</td><td>% G. bulloides</td></tr><tr><td>2490.3</td><td>2449.3</td><td>170.25</td><td>% G. bulloides</td></tr><tr><td>2541.3</td><td>2500.3</td><td>174.63</td><td>% G. bulloides</td></tr><tr><td>2561.5</td><td>2520.5</td><td>178.19</td><td>% G. bulloides</td></tr><tr><td>2590.3</td><td>2549.3</td><td>180.87</td><td>% G. bulloides</td></tr><tr><td>2682.3</td><td>2641.3</td><td>190.12</td><td>% G. bulloides</td></tr><tr><td>2730.0</td><td>2689.0</td><td>195.26</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2777.0</td><td>2736.0</td><td>201.88</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2805.3</td><td>2764.3</td><td>205.10</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2887.8</td><td>2846.8</td><td>217.11</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2915.7</td><td>2874.7</td><td>224.62</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2939.9</td><td>2898.9</td><td>227.66</td><td>% N. pachyderma (sin.)</td></tr><tr><td>2948.2</td><td>2907.2</td><td>229.89</td><td>% N. pachyderma (sin.)</td></tr><tr><td>3000.1</td><td>2959.1</td><td>237.27</td><td>% N. pachyderma (sin.)</td></tr><tr><td>3039.5</td><td>2998.5</td><td>243.07</td><td>% N. pachyderma (sin.)</td></tr><tr><td>3101.5</td><td>3060.5</td><td>247.81</td><td>% N. pachyderma (sin.)</td></tr></tbody></table>
<figure><img src="image_2.png" /></figure>

Africa

20°S

25°S

30°S

35°S

40°S

45°S

50°S
<figure><img src="image_3.png" /></figure>

Age (ka)

% N. pachyderma (sin.)

% N. pachyderma (sin.)
<figure><img src="image_4.png" /></figure>

Laschamp

Iceland Basin
MAD (°)

Declination (°)

Inclination (°)

<figure><img src="image_5.png" /></figure>
(a) Discrete Samples
Laschamp excursion

<figure><img src="image_6.png" /></figure>

3.99 mbsf
(40.33 ka)

<figure><img src="image_7.png" /></figure>

4.12 mbsf
(40.83 ka)

<figure><img src="image_8.png" /></figure>

4.14 mbsf
(40.94 ka)

<figure><img src="image_9.png" /></figure>

4.32 mbsf
(41.77 ka)

(b) Discrete Samples
Iceland Basin excursion

<figure><img src="image_10.png" /></figure>

26.36 mbsf
(185.48 ka)

<figure><img src="image_11.png" /></figure>

26.53 mbsf
(187.13 ka)

<figure><img src="image_12.png" /></figure>

26.56 mbsf
(187.58 ka)

<figure><img src="image_13.png" /></figure>

26.73 mbsf
(189.25 ka)

(c) U-channel Samples

<figure><img src="image_14.png" /></figure>

Laschamp
4.20 mbsf
(41.22 ka)

<figure><img src="image_15.png" /></figure>

Laschamp
4.23 mbsf
(41.39 ka)

<figure><img src="image_16.png" /></figure>

Iceland Basin
26.57 mbsf
(187.45 ka)

<figure><img src="image_17.png" /></figure>

Iceland Basin
26.61 mbsf
(187.92 ka)
<figure><img src="image_18.png" /></figure>
<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>
<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>
<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>
Highlights

(1) Rare recording of Laschamp and Iceland Basin excursions in the South Atlantic.

(2) Age model links excursions to Antarctic ice core and cosmogenic nuclide flux.

(3) South Atlantic ages consistent with North Atlantic ages for the excursions.

(4) No evidence of global diachroneity of magnetic excursions.

(5) Estimated mid-point ages (durations) are 40.9 ka (1 kyr) and 188 kyr (3.5 kyr).