parison with other acquired logs. Borehole shape is excellent through most of Hole U1554F and in agreement with bit size, with only short intervals of widening to a maximum of ~14 inches at ~642 and 644 m WMSF from FMS caliper and to a maximum of ~17 inches over the interval 640–647 m WMSF from the triple combo hydraulic caliper (Figure F57). These good borehole conditions

<figure><img src="image_60.png" /><figcaption>**Figure F56.** Thermal gradient and heat flow estimates, Site U1554. A. Projected APCT-3 formation temperature (open circles), Hole U1554E. Gray line = linear least-squares fit, *T(z)* = thermal gradient. B. Observed thermal conductivity, Holes U1554E and U1554H. C. Calculated thermal resistance (R) (black dots). Gray line = linear least-squares fit for resistance as a function of depth *R(z)*. D. Temperature as a function of thermal resistance (black dots) (i.e., Bullard plot). Gray line = linear least-squares fit, the gradient of which is vertical heat flow (*q*).</figcaption></figure>

<table><caption>**Table T13.** Logging passes, Holes 395C-U1554E and 395C-U1554F. APCT-3 measurements use the DSF depth scale. NA = not applicable. **Download table in CSV format.**</caption><thead><tr><th>Hole</th><th>Tool string</th><th>Tool list</th><th>Logging pass</th><th>Logging data reference</th><th>From depth WSF (m)</th><th>To depth WSF (m)</th><th>Pipe depth WSF (m)</th><th>Notes</th></tr></thead><tbody><tr><td rowspan="6">395C-U1554E</td><td rowspan="3">Triple combo</td><td rowspan="3">MSS-B/HRLA/APS/HLDS/HNGS</td><td>Downlog</td><td>033PUP</td><td>0</td><td>647</td><td>~70.7</td><td>Reference pass, caliper closed</td></tr><tr><td>Repeat (uplog)</td><td>008LUP</td><td>527</td><td>647</td><td>~70.7</td><td>Recorded open hole</td></tr><tr><td>Main (uplog)</td><td>009LUP</td><td>0</td><td>647</td><td>~62.8</td><td></td></tr><tr><td rowspan="3">FMS-sonic</td><td rowspan="3">FMS/DSI-2/GPIT/HNGS</td><td>Downlog</td><td>028PUP</td><td>0</td><td>647</td><td>~72</td><td>Caliper closed</td></tr><tr><td>Repeat (uplog)</td><td>031PUP</td><td>122</td><td>647</td><td>~72</td><td>Recorded open hole</td></tr><tr><td>Main (uplog)</td><td>030PUP</td><td>0</td><td>647</td><td>~62</td><td></td></tr><tr><td>Formation temperature</td><td>APCT-3</td><td></td><td>NA</td><td>NA</td><td>66</td><td>161</td><td>4 measurements (Cores 2H, 5H, 8H, and 11H)</td></tr><tr><td rowspan="8">U1554F</td><td rowspan="3">Triple combo</td><td rowspan="3">MSS-B/HRLA/APS/HLDS/HNGS</td><td>Downlog</td><td>007LDP</td><td>0</td><td>764</td><td>590</td><td>Casing at 603.5 WSF (m), caliper closed</td></tr><tr><td>Repeat (uplog)</td><td>008LUP</td><td>655</td><td>781</td><td>590</td><td>Casing at 603.5 WSF (m)</td></tr><tr><td>Main (uplog)</td><td>009LUP</td><td>0</td><td>781</td><td>590</td><td>Casing at 603.5 WSF (m), reference pass</td></tr><tr><td rowspan="3">FMS-sonic</td><td rowspan="3">FMS/DSI-2/GPIT/HNGS</td><td>Downlog</td><td>029PUP</td><td>0</td><td>782</td><td>590</td><td>Casing at 603.5 WSF (m), caliper closed</td></tr><tr><td>Repeat (uplog)</td><td>030PUP</td><td>650.5</td><td>781</td><td>590</td><td>Casing at 603.5 WSF (m), recorded open hole</td></tr><tr><td>Main (uplog)</td><td>031PUP</td><td>0</td><td>781</td><td>590</td><td>Casing at 603.5 WSF (m)</td></tr><tr><td rowspan="4">UBI</td><td rowspan="4">UBI/GPIT/HNGS</td><td>Downlog</td><td>071PUP</td><td>0</td><td>776</td><td>590</td><td>No image, casing at 603.5 WSF (m)</td></tr><tr><td>Repeat (uplog)</td><td>047LUP</td><td>631</td><td>775</td><td>590</td><td>Recorded open hole, casing at 603.5 WSF (m)</td></tr><tr><td>Repeat (uplog)</td><td>048LUP</td><td>586</td><td>775</td><td>590</td><td>Recorded open hole, casing at 603.5 WSF (m)</td></tr><tr><td>Uplog</td><td>049PUP</td><td>0</td><td>584</td><td>590</td><td>No image, casing at 603.5 WSF (m)</td></tr></tbody></table>
suggest a good degree of reliability in data produced by tools that require eccentricization and good contact with the borehole wall (i.e., APS, HLDS, and FMS), with only the interval 640–647 m WMSF as a possible exception.

Both the FMS and UBI image logs are of good quality, reduced only in a few places by some stick and pull artifacts predominantly on the UBI image log. An approximately 0.8 m offset between similar geologic features on the UBI and FMS image logs is observed, likely due to uncertainty in the image log processing, such as the application of wireline speed corrections. This issue should be considered when correlating between these image logs postcruise.

<figure><img src="image_61.png" /><figcaption>**Figure F57.** Downhole logging data, Hole U1554F. in = inches.</figcaption></figure>
### 11.1.3. Hole U1554H

Downhole measurements acquired in Hole U1554H were formation temperatures (Cores 4H, 7H, 10H, 13H, and 16H). The values increase downhole from 4.35° to 7.77°C (Figure F56).

## 11.2. Logging units

Here we define the logging units for Site U1554 and suggest tentative correlations across Holes U1554E and U1554F where possible (Table T14).

### 11.2.1. Logging Unit L1

Depth: 0–108 m WMSF

Logging Unit L1 is defined from the top of Hole U1554E (seafloor) to where a marked increase in the gamma log occurs at ~108 m WMSF.

#### 11.2.1.1. Logging Subunit L1a

Depth: 0–75 m WMSF

Logging Subunit L1a is defined solely by the suppressed gamma log measurements (maximum = 5 gAPI) resulting from the tool measuring through the drill pipe. Despite measuring through the drill pipe, the gamma log signal shows distinct variations that may be interpretable. Subunit L1a corresponds to the upper part of Lithostratigraphic Subunit IA (0–75 m CSF-A) (see Lithostratigraphy).

#### 11.2.1.2. Logging Subunit L1b

Depth: 75–108 m WMSF

Logging Subunit L1b comprises the open hole portion of Unit L1 and is defined by low gamma log (~10–25 gAPI) that shows a gradual increase with depth toward the bottom of the subunit. The Th spectral gamma measurement fluctuates in this subunit and can be as high as 2.8 ppm, and the U and K contents are consistently low at around 0.2–0.8 ppm and 0.2%–0.5%, respectively. The deep resistivity log for Subunit L1a records slightly higher (~1 Ωm) measurements than the shallow (0.5 Ωm) and medium (0.7 Ωm) logs, and all three resistivity log measurements are invariant throughout the subunit. Subunit L1b is further characterized by porosities fluctuating from 0.6 to 0.9 and an average density of around 1.4 ± 0.1 g/cm³. The MS log shows overall lower values compared to Unit L2 and contains a number of marked peaks across three depth intervals: ~77–82 (468 SI), ~84 (344 SI), and ~90–96 (303 SI) m WMSF. This logging subunit corresponds to the lower portion of Lithostratigraphic Subunit IA (75–108 m CSF-A).

#### 11.2.2. Logging Unit L2

Depth: 108–440 m WMSF

<table><caption>**Table T14.** Downhole logging units, Site U1554. Download table in CSV format.</caption><thead><tr><th rowspan="2">Logging unit</th><th colspan="2">Expedition 395C</th><th rowspan="2">Lithology</th></tr><tr><th>Depth WMSF (m)</th><th>Depth WMSF (m)</th></tr></thead><tbody><tr><td>L1a</td><td>395C-U1554E-0-75</td><td>395C-U1554F-0-75</td><td>Sediment</td></tr><tr><td>L1b</td><td>395C-U1554E-75-108</td><td>395C-U1554F-75-108</td><td>Sediment</td></tr><tr><td>L2a</td><td>395C-U1554E-108-375</td><td>395C-U1554F-108-375</td><td>Sediment</td></tr><tr><td>L2b</td><td>395C-U1554E-375-440</td><td>395C-U1554F-375-440</td><td>Sediment</td></tr><tr><td>L3</td><td>395C-U1554E-440-495</td><td>395C-U1554F-440-495</td><td>Sediment</td></tr><tr><td>L4a</td><td>395C-U1554E-495-535</td><td>395C-U1554F-495-535</td><td>Sediment</td></tr><tr><td>L4b</td><td>395C-U1554E-535-635</td><td>395C-U1554F-535-635</td><td>Sediment</td></tr><tr><td>L4c</td><td>395C-U1554E-635-647</td><td>395C-U1554F-635-647</td><td>Sediment</td></tr><tr><td>L5</td><td>395C-U1554E-647-770</td><td>395C-U1554F-647-770</td><td>Basalt</td></tr></tbody></table>
The top and bottom of Logging Unit L2 are marked by an increase and decrease, respectively, in the gamma log. Unit L2 is further defined by a series of 1–10 m cycles in the gamma log. Peaks in the gamma measurements are mostly comprised of Th signals, as noted from the spectral gamma logs.

#### 11.2.2.1. Logging Subunit L2a

Depth: 108–375 m WMSF

Logging Subunit L2a is defined by an average gamma log response of 27 ± 5 gAPI and shows cycles of distinct 1–10 m long gamma radiation peaks that can increase up to 49 gAPI. These gamma radiation peaks are dominated by a Th signal up to 4.5 ppm, as shown on the spectral gamma logs. Either or both of the K and U counts are also observed to increase in the high Th peaks but to a lesser extent. Electrical resistivity measurements show large fluctuations to ~150 m WSF, below which they are relatively invariant to the bottom of Subunit L2a, with typical deep resistivity of ~1.1 Ωm. The deep resistivity is consistently ~0.2 and ~0.4 Ωm higher than the medium and shallow resistivity measurements, respectively, in Subunit L2a. Porosity fluctuates from 0.7 to 1 to ~150 m WSF and then gradually decreases from ~0.9 to 0.7 to ~250 m WSF, after which they remain ~0.7 to the bottom of Subunit L2a. The bulk density gradually increases with depth in Subunit L2a from ~1.4 to ~1.8 g/cm³. $V_p$ increases with depth from ~1604 to ~1825 m/s, and $V_s$ increases from ~353 to ~480 m/s with depth in Subunit L2a. MS fluctuates between 66 SI and 432 SI in a cyclical manner and gradually increases with depth in the top part of the subunit to ~150 m WMSF. Subunit L2a corresponds to the lower part of Lithostratigraphic Subunit IA and to Subunits IB (108–117.4 m CSF-A) and IIA (117.4–375 m CSF-A) (see Lithostratigraphy).

#### 11.2.2.2. Logging Subunit L2b

Depth: 375–440 m WMSF

Logging Subunit L2b is differentiated from Subunit L2a by longer wavelength cycles in the gamma log fluctuations with depth. Gamma peaks are still dominated by Th counts reaching up to 5 ppm. Density and porosity are invariant throughout Subunit L2b, at ~1.8 g/cm³ and ~0.67, respectively. Electrical resistivity is relatively constant throughout the unit (deep resistivity = ~1.2 Ωm) and decreases slightly in the bottom 5 m of the subunit. $V_p$ gradually increases from ~1750 to ~1820 m/s, and $V_s$ increases from ~440 to ~540 m/s with depth. MS varies cyclically between 114 and 424 SI as in Subunit L2a, and the wavelength of these cycles increases with depth. This subunit corresponds to Lithostratigraphic Subunit IIB (see Lithostratigraphy).

### 11.2.3. Logging Unit L3

Depth: 440–495 m WMSF

Logging Unit L3 is defined by a decrease in gamma log response relative to Units L2 and L4. The lower gamma log (average = 19.2 ± 2.5 gAPI) is relatively constant over this depth interval, with a minor peak at ~461 m WMSF, in contrast to the cyclic fluctuations observed in Units L2 and L4. The spectral gamma Th counts are also lower than those observed in Units L2 and L4, although there are subtle cycles of high and low values in Unit L3. Electrical resistivity decreases slightly compared to Unit L2 (deep resistivity = ~1.1 Ωm), and bulk density (~1.8 g/cm³) and porosity (~0.65) are relatively constant. $V_p$ and $V_s$ continue to increase with depth in Unit L3 to bottom values of ~1885 and ~560 m/s, respectively. A further defining characteristic of Unit L3 is the progressive lengthening of the MS cycles that begins in Subunit L2b, which at the bottom of Unit L3 change back to shorter wavelengths. This logging unit corresponds to the uppermost 45 m of Lithostratigraphic Unit III (441.1–496 m CSF-A) (see Lithostratigraphy).

### 11.2.4. Logging Unit L4

Depth: 495–647 m WMSF

The top of Logging Unit L4 is marked by an increase in gamma log response (27.7 ± 5.2 gAPI), which remains constant with depth to near the sediment/basement interface at ~647 m WMSF.
**11.2.4.1. Logging Subunit L4a**

Depth: 495–535 m WMSF

Logging Subunit L4a is characterized by increased gamma log relative to Unit L3 (average = 25 ± 11 gAPI), and spectral gamma logs show that Th dominates this gamma radiation signal (values up to 4 ppm). Like the logging units above, Th counts fluctuate cyclically. Electrical resistivity remains at similar values to Unit L3 and is relatively constant with depth, as do density and porosity. $V_p$ increases slightly with depth to ~1950 m/s, and $V_s$ increases slightly to ~600 m/s. MS is broadly similar to that in Units L2 and L3, although the spacing between peaks and troughs is reduced compared to Unit L3 and appears similar to MS cycles observed throughout Unit L2. This subunit corresponds to the lower part of Lithostratigraphic Unit III (495–538.1 m CSF-A) (see Lithostratigraphy).

**11.2.4.2. Logging Subunit L4b**

Depth: 535–635 m WMSF

The transition from Logging Subunit L4a to Logging Subunit L4b is marked by small but sharp decreases in resistivity, density, porosity, $V_s$, and MS and a small but sharp increase in $V_p$, as defined on wireline logs from Hole U1554E (Figure F58). The transition between Subunits L4b and L4c is defined by a small increase in resistivity, a decrease in porosity, increases in density and $V_p$, and a less pronounced increase in $V_s$, as shown in wireline logs from Hole U1554F (Figure F57).

The gamma log in Subunit L4b is similar to that in Subunit L4a with similar cycles of high and low gamma radiation throughout. Although the spectral gamma logs show Th remains the dominant component of the gamma signal in Subunit L4b, one specific gamma radiation peak at ~627 m WMSF is dominated by a higher U signal. This U-dominated gamma peak corresponds to a slight decrease in resistivity, an increase in porosity (up to 0.86), and a decrease in density (to ~1.44 g/cm³) and is also observed in physical properties data collected on the core from Hole U1554F (see Physical properties). After an initial resistivity low at the top of Subunit L4b, resistivity increases and becomes consistent again at ~550 m WMSF (deep resistivity of ~1.1 Ωm). Similarly, porosity decreases and density increases with depth from the top of Subunit L4b, becoming invariant with depth from ~560 m WMSF at ~0.6 and 1.8 g/cm³, respectively. $V_p$ and $V_s$ increase rapidly from the top of Subunit L4b to ~550 m WMSF, after which they fluctuate with depth to the bottom of Subunit L4b ($V_p$ ranges 1720–1900 m/s; $V_s$ ranges 510–730 m/s). MS in Subunit L4b shows an overall decrease compared to the upper logging units, ranging 50–440 SI but still displaying similar wavelength cycles to those observed in Unit L2. Apart from the outlier noted above, porosity decreases with depth (to 0.3) and density increases (to 2.2 g/cm³). Subunit L4b correlates to Lithostratigraphic Unit IV (538.4–635.4 m CSF-A) (see Lithostratigraphy).

**11.2.4.3. Logging Subunit L4c**

Depth: 635–647 m WMSF

Logging Subunit L4c comprises the bottom portion of the sediment package that sits above the basaltic basement of Site U1554 and contains distinct logging responses (Figure F57). The gamma log is low throughout Subunit L4c (range = 5–11 gAPI) and is dominated by a Th component. Resistivity is slightly higher than Subunit L4b sediments and is consistent in Subunit L4c, showing values between 1 and 2.5 Ωm. Porosity and density fluctuate in Subunit L4c with increases in density paired with decreases in porosity and vice versa. Density is as high as ~2.2 g/cm³ in Subunit L4c, and porosity is as low as 0.29. At the sediment/basement interface, porosity increases to nearly 1 and density decreases to ~1.2 g/cm³. $V_p$ increases rapidly with depth in Subunit L4c from 1740 to 4490 m/s, with some intervals of increased $V_p$ at ~638 and ~642 m WMSF. $V_s$ also increases with depth in Subunit L4c (from 578 to 1152 m/s). The MS log changes from fluctuating in Subunit L4b to being more constant in this subunit (average = 28 ± 8 SI).

**11.2.5. Logging Unit L5**

Depth: 647–780 m WMSF
Logging Unit L5 is distinguished by marked changes in all downhole logs and represents the entire basement section cored in Hole U1554F. Gamma log values are much higher than in the sediment units (average = 14 ± 3 gAPI). Small fluctuations on the gamma log are attributed to increasing and decreasing Th counts in the signal. A small increase in spectral gamma U is noted from ~730 m WSF to the bottom of Hole U1554F at 770 m WSF. Resistivity is also significantly higher than in the logging units above, with deep resistivity ranging ~20–1650 Ωm. Resistivity logs show variation on the meter scale, with intervals of increased resistivity often showing separation between the shallow and medium/deep resistivity measurements (e.g., ~702 m WMSF). Porosity is unsurprisingly much lower in Unit L5 than in the sedimentary units above, with values <0.2 for most of

<figure><img src="image_62.png" /><figcaption>**Figure F58.** Downhole logging data, Hole U1554E. in = inches.</figcaption></figure>
the drilled basement section. As expected, density is higher in Unit L5 than in the sedimentary units above, and is mostly >2.7 g/m³ throughout. Throughout Unit L5, the density log shows short intervals (<1 m) where values decrease below ~2.7 g/cm³, which are often, but not always, accompanied by increases in porosity to >0.2 (e.g., 690 m WMSF). Vp and Vs gradually increase in the top 3 m of basement, reaching averages of 5400 ± 340 m/s and 2870 ± 220 m/s, respectively. Fluctuations in both Vs and Vp are observable in Unit L5 and often occur over the same depth intervals (e.g., ~719 m WMSF). The top of Unit L5 is marked by an increase in MS to a background level of ~200 SI. MS also shows frequent meter-scale increases up to 1030 SI, some of which correlate to the previously mentioned meter-scale resistivity increases.

**11.3. Borehole image logging**

FMS image logs were acquired in sediment sections from Hole U1554E and basement sections from Hole U1554F (Figures F59, F60). An acoustic image log was also acquired for the drilled basement section of Hole U1554F (Figure F60). Excellent image quality allows a range of geologic features to be described and measured, including sedimentary bedding, lithologic texture, and fractures.

The resistivity image log from Hole U1554E covers the depth interval containing Logging Subunits L2a–L4c (~125–642 m WMSF). Unit L2 is characterized by closely spaced, decimeter-scale, gently dipping alternating resistive and conductive sedimentary layers. Unit L3 has slightly thicker layering than Unit L2, and layers have increased amplitude, suggesting a higher angle of dip here. Unit L4 shows gently dipping sedimentary layers with thin conductive layers interbedded with decimeter-thick resistive layers, which in places appear fractured. From the top of Subunit L4b to ~552 m WMSF, resistivity images show a number of centimeter- to decimeter-sized conductive or resistive patches that may be clasts (Figure F61A). Acoustic and resistivity image logs from Hole U1554F were acquired entirely within Unit L5 (basement) at ~651–775 (acoustic image log)

<figure><img src="image_63.png" /><figcaption>**Figure F59.** Wireline image data, Hole U1554E. in = inches.</figcaption></figure>
and ~651–780 m WMSF (resistivity image log). Acoustic image logs show both high- and low-amplitude image facies, with the low-amplitude image facies containing multiple fractures and what may be small-scale pillow structures (Figure F61B). Similarly, resistivity images of the basement basalts show alternating bands of high and low resistivity crosscut by a complex set of resistive and conductive fractures.

<figure><img src="image_64.png" /><figcaption>**Figure F60.** Wireline image data, Hole U1554F. in = inches.</figcaption></figure>
**11.4. Downhole temperature and heat flow**

Four APCT-3 tool measurements were made in Hole U1554E, and five were made in Hole U1554H. Data from Hole U1554E appear reliable (Figure F62A); however, observed heat pulse decays for Cores 395C-U1554H-4H and 7H contain spikes, likely due to increased heave causing motion of the APC cutting shoe during measurement (Figure F62B). As a result, downhole tem-

<figure><img src="image_65.png" /></figure>

**Figure F61.** Conductive features, Site U1554. A. FMS resistivity image log at 543–546 m WMSF highlighting conductive clasts with resistive halos, Hole U1554E. B. Acoustic amplitude and traveltime image logs at 750–755 m WMSF showing alternating high (brighter) and low (darker) amplitude layering in the basement, conductive fractures, and conductive structures that may represent meter-scale pillow basalt structures, Hole U1554F.

<figure><img src="image_66.png" /></figure>

**Figure F62.** APCT-3 temperature-time series and projected formation temperatures, Holes (A) U1554E and (B) U1554H. Colored lines = observed temperature-time series, dashed lines = modeled heat pulse decay function (Hartmann and Villinger, 2002), colored dots = formation temperature projected using the best-fitting heat pulse decay function. Time is normalized with respect to onset of the heat pulse decay model.
<table><caption>**Table T15.** APCT-3 measurements, Hole 395C-U1554E. Download table in CSV format.</caption><thead><tr><th>Section</th><th>Depth<br>DSF (m)</th><th>Formation temperature<br>(°C)</th></tr></thead><tbody><tr><td>395C-U1554E-</td><td></td><td></td></tr><tr><td>2H</td><td>75.80</td><td>5.59</td></tr><tr><td>5H</td><td>104.30</td><td>6.48</td></tr><tr><td>8H</td><td>132.80</td><td>7.17</td></tr><tr><td>11H</td><td>161.30</td><td>8.25</td></tr></tbody></table>

perature measurements from U1554H are considered unreliable. Projected downhole formation temperatures in Hole U1554E range from 5.6°C at 75.8 m DSF to 8.3°C at 161.3 m DSF (Figures F56A, F62A; Table T15), giving a geothermal gradient of 30°C/km. Thermal conductivity under in situ conditions, K, was estimated from laboratory-determined thermal conductivity from Holes U1554E and U1554H using the method of Hyndman et al. (1974) (Figure F56B). A linear function for depth-dependent thermal resistance (R) was obtained by integrating the inverse of the thermal conductivity over depth and applying a linear least-squares fit (Figure F56C). This function was used to estimate thermal resistance at each corresponding APCT-3 measurement depth. A heat flow of 28.6 mW/m² was obtained from a linear least-squares fit of the relationship between temperature and thermal resistance (Bullard, 1939), whose slope is the vertical heat flow (Figure F56D).

## 12. Age model

Site U1554 is located near the center of Björn drift where the sedimentary succession is near its thickest on site survey Seismic Profile JC50-1 (see **Background and objectives**). The site survey profiles show acoustically continuous reflectors through most of the succession from the seafloor to ~3.2 s TWT (Figure F2). Between that level and a clear basement reflection at ~3.3 s TWT, the seismic reflection profiles show greater complexity, including less continuous reflections and some apparent angular discordance of reflections along the profiles. In comparison to Site U1562, which was drilled near the eastern edge of Björn drift, the upper part of the succession above ~3.2 s TWT appears more expanded and the lower part is more condensed (Figure F2).

Age model tie points and interpolated boundaries at Site U1554 are provided in Table T16 and Figure F63. The age model is derived mainly from the succession in Hole 395C-U1554E, which was cored continuously from the seafloor to the basement at 645.20 m CSF-A, with additional biostratigraphic constraints from Hole 395-U1554G. From the seafloor to the base of Matuyama Subchron C2r.2r at 402.34 m CSF-A, the age model is based on magnetostratigraphic reversals (see **Paleomagnetism**) complemented by calcareous nannofossil and planktonic foraminifer biostratigraphy (see **Micropaleontology**). Below that depth, a succession of magnetic polarity reversals was recorded, but their magnetostratigraphic interpretation is highly uncertain because of gaps in the record, so the age model relies on biostratigraphy alone, which is based on calcareous nannofossils, planktonic foraminifers, and bolboforms.

From the seafloor to the base of Chron C2r.2r (2.595 Ma) at 402.34 m CSF-A, the age model is straightforward and well constrained. Some biostratigraphic constraints deviate slightly from the linear model fit, but these can be explained by taxonomic uncertainties or possible diachrony between the calibration sites and this relatively high latitude location (see **Micropaleontology**). The data indicate continuous, high-rate sedimentation. The average sedimentation rate for Lithostratigraphic Subunit IA, from the seafloor to 118 m CSF-A (see **Lithostratigraphy**) is ~13 cm/ky. The average sedimentation rate for Subunit IB and Unit II combined is 17 cm/ky. These sedimentation rates are consistent with a fast-accumulating contourite drift deposit.

For the lower part of the succession from 560 m CSF-A to basement, we present a simple linear age model using two relatively well defined planktonic foraminifer bioevents (Base *G. puncticulata* at 560 m CSF-A and top *P. mayeri* at 627 m CSF-A). The average accumulation rate for this part of the succession is much lower than the overlying interval, at ~0.9 cm/ky. There may be cryptic hia-
tuses, as evidenced by two bolboform age estimates that lie slightly off the best fit model, but we cannot discount diachrony because the bolboform assemblages at this site differ from the main calibration site, Ocean Drilling Program (ODP) Site 982 in the Hatton-Rockall Basin (Spiegler, 1999) (see Micropaleontology). The biostratigraphic data are therefore not considered suffi-

<table><caption>**Table T16.** Age model tie points and interpolated boundaries, Hole 395C-U1554E. Download table in CSV format.</caption><thead><tr><th>Tie point</th><th>Event (base)</th><th>Age (Ma)</th><th>Depth CSF-A (m)</th></tr></thead><tbody><tr><td>1</td><td>Top of hole</td><td>0.000</td><td>0.00</td></tr><tr><td>2</td><td>C1n (Brunhes)</td><td>0.773</td><td>100.72</td></tr><tr><td></td><td>Lithostratigraphic Subunit IA</td><td>0.904</td><td>118.30</td></tr><tr><td>3</td><td>C1r.1n (Jaramillo)</td><td>1.070</td><td>140.72</td></tr><tr><td>4</td><td>C1r.2n (Cobb Mountain)</td><td>1.215</td><td>154.27</td></tr><tr><td></td><td>Lithostratigraphic Subunit IB</td><td>1.764</td><td>240.30</td></tr><tr><td>5</td><td>C2n (Olduvai)</td><td>1.934</td><td>266.98</td></tr><tr><td></td><td>Lithostratigraphic Subunit IIA</td><td>2.45</td><td>373.50</td></tr><tr><td></td><td>Base Pleistocene</td><td>2.59</td><td>401.32</td></tr><tr><td>6</td><td>C2r.2r (Matuyama)</td><td>2.595</td><td>402.34</td></tr><tr><td></td><td>Lithostratigraphic Subunit IIB (extrapolated down)</td><td>2.79</td><td>441.40</td></tr><tr><td></td><td>Lithostratigraphic Unit III (extrapolated down)</td><td>3.26</td><td>538.40</td></tr><tr><td>7</td><td>Base <i>Globoconella puncticulata</i></td><td>4.54</td><td>559.89</td></tr><tr><td></td><td>Base Pliocene</td><td>5.33</td><td>568.74</td></tr><tr><td>8</td><td>Top <i>Paragloborotalia mayeri</i></td><td>10.54</td><td>627.12</td></tr><tr><td></td><td>Base upper Miocene</td><td>11.65</td><td>639.56</td></tr><tr><td></td><td>Lithostratigraphic Subunit IVB (extrapolated down)</td><td>12.16</td><td>645.23</td></tr></tbody></table>

<figure><img src="image_67.png" /><figcaption>**Figure F63.** Preliminary age model for the sedimentary succession, Hole U1554E. Additional biostratigraphic constraints from Hole U1554G are shown. Note that downhole logging units are on the WMSF depth scale, for which there is a small offset (not shown) relative to the CSF-A scale.</figcaption></figure>
ciently reliable to support firm conclusions without clear lithologic or cyclostratigraphic evidence of hiatuses. The interval is characterized by a series of magnetic polarity reversals, but because of the low sedimentation rate the magnetostratigraphic interpretation remains preliminary (see **Paleomagnetism**).

The most uncertain part of the age model is between ~440 and 552 m CSF-A, which corresponds to Lithostratigraphic Unit III and the upper part of Subunit IVA. Downhole logging data show a sharp decrease in bulk density at ~535 m WMSF, which corresponds to the top of Logging Unit L3b (see **Downhole measurements**) and is at a similar level to the base of Lithostratigraphic Unit III at ~538 m CSF-A (see **Lithostratigraphy**). Two clasts were observed embedded in the clay- or silt-rich sediment core, one apparently of basalt ~7 cm in diameter in Section 395C-U1554E-50X-2 and one of relatively angular sedimentary rock ~3 cm in diameter in Section 52X-3 (see **Lithostratigraphy**). Basalt pebbles were also observed at the top of Core 51X. The stratigraphic interval immediately above this interval (Cores 48X-50X; 511-532 m CSF-A) shows evidence of winnowing, concentrating glauconite, and reworking of middle Miocene microfossils from levels equivalent to the lowermost part of the stratigraphic succession at the site (see **Micropaleontology**).

In this interval, within which there is a substantial change in the long-term sedimentation rate, we take a conservative approach by extrapolating the relatively well constrained sedimentation rate from higher in the succession to 535 m CSF-A but consider this part of the age model to carry significant uncertainty (Figure F63).

Igneous basement in Hole 395C-U1554F was encountered at 645.23 m CSF-A. The age of the oldest sediment above this, extrapolated downward using the linear age model for the lower part of the sedimentary succession, is ~12.16 Ma. This age is slightly younger than the age of the basement as estimated from regional magnetic anomaly data of 12.68 Ma (see **Background and objectives**).

## References

Alken, P., Thébault, E., Beggan, C.D., Amit, H., Aubert, J., Baerenzung, J., Bondar, T.N., Brown, W.J., Calififf, S., Cham-bodut, A., Chulliat, A., Cox, G.A., Finlay, C.C., Fournier, A., Gillet, N., Grayver, A., Hammer, M.D., Holschneider, M., Huder, L., Hulot, G., Jager, T., Kloss, C., Korte, M., Kuang, W., Kuvshinov, A., Langlais, B., Léger, J.M., Lesur, V., Livermore, P.W., Lowes, F.J., Macmillan, S., Magnes, W., Mandea, M., Marsal, S., Matzka, J., Metman, M.C., Min-ami, T., Morschhauser, A., Mound, J.E., Nair, M., Nakano, S., Olsen, N., Pavón-Carrasco, F.J., Petrov, V.G., Ropp, G., Rother, M., Sabaka, T.J., Sanchez, S., Saturnino, D., Schnepf, N.R., Shen, X., Stolle, C., Tangborn, A., Tøffner-Clau-sen, L., Toh, H., Torta, J.M., Varner, J., Vervelidou, F., Vigneron, P., Wardinski, I., Wicht, J., Woods, A., Yang, Y., Zeren, Z., and Zhou, B., 2021. International Geomagnetic Reference Field: the thirteenth generation. Earth, Planets and Space, 73(1):49. https://doi.org/10.1186/s40623-020-01288-x

Berggren, W.A., 1972. Cenozoic biostratigraphy and paleobiogeography of the North Atlantic. In Laughton, A.S., Berg-gren, W.A., et al., Initial Reports of the Deep Sea Drilling Project. 12: Washington, DC (US Government Printing Office), 965-1001. https://doi.org/10.2973/dsdp.proc.12.114.1972

Blum, P., Rhinehart, B., and Acton, G.D., 2020. International Ocean Discovery Program Expedition 384 Preliminary Report. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.384.2020

Bullard, E.C., 1939. Heat flow in South Africa. Proceedings of the Royal Society of London, A: Mathematical and Physical Sciences, 173(955):474-502. https://doi.org/10.1098/rspa.1939.0159

Butterworth, S., 1930. On the theory of filter amplifiers. Experimental Wireless and the Wireless Engineer, 7:536-541. https://www.gonascent.com/papers/butter.pdf

Colman, A., Sinton, J.M., White, S.M., McClinton, J.T., Bowles, J.A., Rubin, K.H., Behn, M.D., Cushman, B., Eason, D.E., Gregg, T.K.P., Grönvold, K., Hidalgo, S., Howell, J., Neill, O., and Russo, C., 2012. Effects of variable magma supply on mid-ocean ridge eruptions: constraints from mapped lava flow fields along the Galápagos Spreading Center. Geochemistry, Geophysics, Geosystems, 13(8). https://doi.org/10.1029/2012GC004163

Flower, B.P., 1999. Data report: planktonic foraminifers from the subpolar North Atlantic and Nordic Seas: Sites 980-987 and 907. In Raymo, M.E., Jansen, E., Blum, P., and Herbert, T.D. (Eds.), Proceedings of the Ocean Drilling Program, Scientific Results. 162: College Station, TX (Ocean Drilling Program). https://doi.org/10.2973/odp.proc.sr.162.038.1999

Foresi, L.M., Iaccarino, S.M., and Salvatorini, G., 2002. Neogloboquadrina alantica preatlantica, new subspecies from late Middle Miocene. Rivista Italiana Di Paleontologia E Stratigrafia, 108(2). https://doi.org/10.13130/2039-4942/5479

Hartmann, A., and Villinger, H., 2002. Inversion of marine heat flow measurements by expansion of the temperature decay function. Geophysical Journal International, 148(3):628-636. https://doi.org/10.1046/j.1365-246X.2002.01600.x
Hyndman, R.D., Erickson, A.J., and Von Herzen, R.P., 1974. Geothermal measurements on DSDP Leg 26. In Davies, T.A., Luyendyk, B.P., et al., Initial Reports of the Deep Sea Drilling Project. 26: Washington, DC (US Government Printing Office), 451–463. https://doi.org/10.2973/dsdp.proc.26.113.1974

Lurcock, P.C., and Florindo, F., 2019. New developments in the PuffinPlot paleomagnetic data analysis program. Geochemistry, Geophysics, Geosystems, 20(11):5578–5587. https://doi.org/10.1029/2019GC008537

Lyons, R.G., 2011. Understanding Digital Signal Processing (Third edition): Upper Saddle River, NJ (Prentice Hall). https://www.iro.umontreal.ca/~mignotte/IFT3205/Documents/UnderstandingDigitalSignalProcessing.pdf

Murton, B.J., Taylor, R.N., and Thirlwall, M.F., 2002. Plume-ridge Interaction: a geochemical perspective from the Reykjanes Ridge. Journal of Petrology, 43(11):1987–2012. https://doi.org/10.1093/petrology/43.11.1987

Ogg, J.G., 2020. Geomagnetic Polarity Time Scale. In Gradstein, F.M., Ogg, J.G., Schmitz, M., and Ogg, G. (Eds.), Geo-logic Time Scale 2020. Amsterdam (Elsevier), 159–192. https://doi.org/10.1016/B978-0-12-824360-2.00005-X

Parnell-Turner, R.E., Briais, A., LeVay, L.J., Cui, Y., Di Chiara, A., Dodd, J.P., Dunkley Jones, T., Dwyer, D., Eason, D.E., Friedman, S.A., Hemming, S.R., Hochmuth, K., Ibrahim, H., Jasper, C., Karatsolis, B.T., Lee, S., LeBlanc, D.E., Lind-say, M.R., McNamara, D.D., Modestou, S.E., Murton, B., OConnell, S., Pasquet, G.T., Pearson, P.N., Qian, S.P., Rosenthal, Y., Satolli, S., Sinnesael, M., Suzuki, T., Thulasi Doss, T., White, N.J., Wu, T., and Yang Yang, A., 2025a. Expedition 395 methods. In Parnell-Turner, R.E., Briais, A., LeVay, L.J., and the Expedition 395 Scientists, Reyk-janes Mantle Convection and Climate. Proceedings of the International Ocean Discovery Program, 395: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.395.102.2025

Parnell-Turner, R.E., Briais, A., LeVay, L.J., Cui, Y., Di Chiara, A., Dodd, J.P., Dunkley Jones, T., Dwyer, D., Eason, D.E., Friedman, S.A., Hemming, S.R., Hochmuth, K., Ibrahim, H., Jasper, C., Karatsolis, B.T., Lee, S., LeBlanc, D.E., Lind-say, M.R., McNamara, D.D., Modestou, S.E., Murton, B., OConnell, S., Pasquet, G.T., Pearson, P.N., Qian, S.P., Rosenthal, Y., Satolli, S., Sinnesael, M., Suzuki, T., Thulasi Doss, T., White, N.J., Wu, T., Yang Yang, A., dos Santos Rocha, V., Pearman, C., and Tian, C.Y., 2025b. Expedition 395 summary. In Parnell-Turner, R.E., Briais, A., LeVay, L.J., and the Expedition 395 Scientists, Reykjanes Mantle Convection and Climate. Proceedings of the International Ocean Discovery Program, 395: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.395.101.2025

Parnell-Turner, R.E., Briais, A., LeVay, L.J., and the Expedition 395 Scientists, 2025c. Supplementary material, https://doi.org/10.14379/iodp.proc.395supp.2025. In Parnell-Turner, R.E., Briais, A., LeVay, L.J., and the Expe-dition 395 Scientists, Reykjanes Mantle Convection and Climate. Proceedings of the International Ocean Discovery Program, 395: College Station, TX (International Ocean Discovery Program).

Parnell-Turner, R., White, N., Henstock, T.J., Jones, S.M., Maclennan, J., and Murton, B.J., 2017. Causes and conse-quences of diachronous v-shaped ridges in the North Atlantic Ocean. Journal of Geophysical Research: Solid Earth, 122(11):8675–8708. https://doi.org/10.1002/2017JB014225

Poag, C.W., and Karowe, A.L., 1986. Stratigraphic potential of Bolboforma significantly increased by new finds in the North Atlantic and South Pacific. Palaios, 1(2):162–171. https://doi.org/10.2307/3514510

Poore, R.Z., 1979. Oligocene through Quaternary planktonic foraminiferal biostratigraphy of the North Atlantic: DSDP Leg 49. In Luyendyk, B.P., Cann, J.R., et al., Initial Reports of the Deep Sea Drilling Project. 49: Washington, DC (US Government Printing Office), 447–517. https://doi.org/10.2973/dsdp.proc.49.115.1979

Poore, R.Z., and Berggren, W.A., 1975. Late Cenozoic planktonic foraminiferal biostratigraphy and paleoclimatology of Hatton-Rockall Basin; DSDP Site 116. Journal of Foraminiferal Research, 5(4):270–293. https://doi.org/10.2113/gsjfr.5.4.270

Skilling, I.P., White, J.D.L., and McPhie, J., 2002. Peperite: a review of magma–sediment mingling. Journal of Volcanol-ogy and Geothermal Research, 114(1–2):1–17. https://doi.org/10.1016/S0377-0273(01)00278-5

Spiegler, D., 1999. Bolboforma biostratigraphy from the Hatton-Rockall Basin (North Atlantic). In Raymo, M.E., Jan-sen, E., Blum, P., and Herbert, T.D., Proceedings of the Ocean Drilling Program, Scientific Results, 162. College Station, TX (Ocean Drilling Program), 35–49. https://doi.org/10.2973/iodp.proc.sr.162.013.1999

Weaver, P.P.E., 1987. Late Miocene to recent planktonic foraminifers from the North Atlantic: Deep Sea Drilling Proj-ect Leg 94. In Ruddiman, W.F., Kidd, R. B., Thomas, E., et al., Initial Reports of the Deep Sea Drilling Project. 94: Washington, DC (US Government Printing Office), 703–727. https://doi.org/10.2973/dsdp.proc.94.114.1987

Winter, C.J., and Pearson, P.N., 2001. Coiling directions in some Miocene planktonic Foraminifera. Journal of Micropa-laeontology, 20(1):29–30. https://www.lyellcollection.org/doi/abs/10.1144/jm.20.1.29

Zijderveld, J.D.A., 1967. AC demagnetization of rocks: analysis of results. In Runcorn, S.K.C., Creer, K.M., and Col-linson, D.W. (Eds.), Methods in Palaeomagnetism. Developments in Solid Earth Geophysics, 3: 254–286. https://doi.org/10.1016/B978-1-4832-2894-5.50049-5