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# Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2

Hongjin Chen, Germain Bayon, Zhaokai Xu, Tiegang Li

► To cite this version:

Hongjin Chen, Germain Bayon, Zhaokai Xu, Tiegang Li. Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2. Earth and Planetary Science Letters, 2023, 601, 117910 (10p.). 10.1016/j.epsl.2022.117910. hal-04203926

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Distributed under a Creative Commons Attribution 4.0 International License
1 **Hafnium isotope evidence for enhanced weatherability at high**

2 **southern latitudes during Oceanic Anoxic Event 2**

3

4 **Hongjin Chen<sup>1,2,6</sup>, Germain Bayon<sup>2*</sup>, Zhaokai Xu<sup>1,3,4,5*</sup>, Tiegang Li<sup>7</sup>**

5

6 <sup>1</sup> CAS Key Laboratory of Marine Geology and Environment, Institute of Oceanology,

7 Chinese Academy of Sciences, Qingdao 266071, China

8 <sup>2</sup> Univ Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France

9 <sup>3</sup> Laboratory for Marine Geology, Pilot National Laboratory for Marine Science and

10 Technology (Qingdao), Qingdao 266061, China

11 <sup>4</sup> CAS Center for Excellence in Quaternary Science and Global Change, Xi'an

12 710061, China

13 <sup>5</sup> Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071,

14 China

15 <sup>6</sup> Key Laboratory of Marine Mineral Resources, Ministry of Natural Resources,

16 Guangzhou Marine Geological Survey, China Geological Survey

17 <sup>7</sup> Key Laboratory of Marine Sedimentology and Metallogeny, First Institute of

18 Oceanography, Ministry of Natural Resources, Qingdao 266061, China

19

20

21 Correspondence to Germain Bayon (Germain.Bayon@ifremer.fr) and Zhaokai Xu

22 (zhaokaixu@qdio.ac.cn)
**Abstract:**

The Oceanic Anoxic Event 2 (OAE 2; ca. ~94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO₂ drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (Δε<sub>Hfclay</sub>) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (~62°S) during the late Cretaceous. The reliability of Δε<sub>Hfclay</sub> as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions.

At Site U1516, the early stage of OAE 2 was characterized by relatively low ΔεHfclay values (-10.3 ± 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in ΔεHfclay points towards accelerated export of poorly weathered sediments
resulting from the abrupt reactivation of river systems in southwest Australia. This
period was followed by a pronounced ΔεHfclay shift towards positive values, indicative
of intensifying chemical weathering conditions during the OAE 2 interval showing the
highest δ13C anomaly. Based on these results, we posit that enhanced hydrological
cycle, most likely caused by a southward shift of the westerlies, led to a large increase
in weatherability at southern high latitudes during peak OAE 2 warmth. This finding
provides empirical support for the potential role played by high-latitude weathering
systems in driving the termination of OAE 2, via weathering-driven consumption of
atmospheric CO2 and accelerated riverine fluxes of nutrients leading to enhanced
organic carbon burial in marine sediments.

55

56 **Keywords:** OAE 2; IODP Expedition 369; Mentelle Basin; hafnium-neodymium
57 isotopes; silicate weathering; reverse weathering

58
# 1. Introduction

Understanding the dynamics of the Earth's climate system during past carbon cycle perturbations is essential to improve predictions on future climate change (Huber et al., 2019; Takashima et al., 2006). The Oceanic Anoxic Event 2 (OAE 2), which occurred at the Cenomanian-Turonian boundary (~94.5 to 93.9 Ma), represents the most significant global carbon cycle perturbation of the Late Cretaceous (Arthur et al., 1987; Schlanger and Jenkyns, 1976), characterized by widespread organic carbon sequestration and a pronounced positive carbon isotope excursion (CIE, ~2‰) in the sedimentary record (Jenkyns, 2010; Tsikos et al., 2004). Possible driving mechanisms for OAE 2 have long been debated, but consensus exists that it was closely related to the emplacement of large igneous provinces (LIP) at that time (Fig. 1) (e.g., Turgeon and Creaser, 2008), associated with massive CO₂ release into the atmosphere (O'Brien et al., 2017), enhanced hydrological cycle (van Helmond et al., 2013) and increased marine primary production (Schlanger and Jenkyns, 1976). Following emplacement of LIPs, sustained chemical weathering of freshly exposed mafic rocks under warm and wet conditions most likely represented a major sink for atmospheric CO₂ (e.g., Macdonald et al., 2019), possibly playing a role in the OAE 2 termination and the recovery of the Earth's climate system from greenhouse conditions (e.g., Pogge von Strandmann et al., 2013). Additionally, enhanced global 'weatherability'; a term that refers to the ease with which terrestrial rocks are subject to chemical weathering at the Earth's surface, and implicitly relates to riverine solute fluxes (e.g., Kump et al., 2000; Mills et al., 2014; Penman et al., 2020), presumably resulted in accelerated continental
fluxes of phosphate and other key nutrients to the oceans, thereby contributing to high
rates of primary productivity and organic carbon burial (e.g., Jenkyns, 2010; Pogge
von Strandmann et al., 2013; Yobo et al., 2021). Up to now, the evolution of
continental chemical weathering during OAE 2 and other past global carbon cycle
perturbations of the Phanerozoic has been mostly inferred from the application of
isotope proxies (e.g., Li, Ca, Sr, and Os) to the sedimentary record of ancient seawater
chemistry (Blättler et al., 2011; Bottini et al., 2012; Frijia and Parente, 2008; Pogge
von Strandmann et al., 2013). Changes in continental chemical weathering fluxes
during the OAE 2 have been documented at a very few locations in the Northern
Hemisphere, using Li isotopes in the carbonate record as a proxy for global seawater
chemistry (Pogge von Strandmann et al., 2013). Unlike other marine-based proxies,
the δ<sup>7</sup>Li composition of marine carbonate mostly reflects the global continental flux
and isotopic composition of dissolved riverine Li related to chemical weathering of
silicate rocks (Misra and Froelich, 2012; Pogge von Strandmann et al., 2013).
However, the lack of additional proxy evidence at the regional scale, especially for
high-latitude regions, still hampers our understanding of the way continental chemical
weathering may have responded to rapid climate change during the OAE 2, and
whether chemical weathering contributed to driving the termination of greenhouse
climate state.

100

The combined use of hafnium (Hf) and neodymium (Nd) isotope ratios in the
clay-size fraction of siliciclastic sediments represents a novel promising approach for
reconstructing regional changes in past continental weathering (Bayon et al., 2022; Corentin et al., 2022). Most igneous rocks and terrestrial sediments lie along a single correlation trend in a ¹⁴³Nd/¹⁴⁴Nd (or εNd in epsilon notation) versus ¹⁷⁶Hf/¹⁷⁷Hf (εHf) plot, defined as the ‘Terrestrial Array’ (εHf = 1.55 × εNd + 1.21) (Fig. 2) (Vervoort et al., 2011). While the Nd isotopic composition of fine-grained detrital sediments remains mostly unchanged during continental erosion, sediment transport and deposition (e.g., Bayon et al., 2015), Hf isotopes are strongly affected by preferential sorting of Hf-bearing zircon into coarse sediments during transport processes (e.g., Bayon et al., 2009; Garçon et al., 2013). Because zircons are typically characterized by unradiogenic signatures (with low εHf) and dominate the Hf budget in detrital sediments, sediment transport hence results in zircon-depleted clay-size sediment fractions exhibiting much higher εHf composition than corresponding zircon-bearing silts, defining a so-called ‘Clay Array’ in the εNd versus εHf plot (εHf = 0.78 × εNd + 5.23; Zhao et al., 2014; Bayon et al., 2016). In addition to the ‘zircon effect’, preferential dissolution of accessory Lu-rich minerals such as apatite during continental chemical weathering releases substantial amounts of radiogenic Hf to surface environments (e.g., Bayon et al., 2006; Rickli et al., 2013). This process most likely explains why seawater and river waters typically display radiogenic (high) εHf composition relative to εNd (Fig. 2) (e.g., Bayon et al., 2006; Rickli et al., 2013). Bayon et al. (2016) showed that the decoupling of Hf and Nd isotopes during continental weathering is also recorded in the clay-size fraction of river-borne sediments, demonstrating strong relationships with environmental parameters and the
intensity of chemical weathering. The degree of Hf-Nd isotope decoupling during
chemical weathering can be quantitatively measured using the vertical εHf deviation
from the ‘Clay Array’ (Bayon et al., 2016). This index, termed ΔεHfclay, can serve as a
unique proxy for reconstructing continental weathering and past environmental
conditions over various geological timescales (e.g., Bayon et al., 2022; Corentin et al.,
2022). However, to date, whether this proxy can be successfully applied to sediments
or sedimentary rocks that have experienced intense post-depositional processes such
as reverse weathering and clay authigenesis still remains unclear (Bayon et al., 2022).

133

In this study, we present Hf and Nd isotopic compositions and elemental
geochemistry of clay-size (<2 µm) siliciclastic fractions of fine-grained sediments
deposited at the International Ocean Discovery Program (IODP) Site U1516 during
OAE 2 (Fig. 1), combined with additional Hf-Nd isotope analyses of clays extracted
from methanogenic sediments strongly influenced by marine silicate weathering (i.e.,
reverse weathering). This study aims to examine the potential impact of reverse
weathering on ΔεHfclay and its utility as a weathering proxy, and to investigate how
chemical weathering in southern high latitudes may have responded to greenhouse
climate change during OAE 2.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** a) Paleogeographic map of ~94 Ma showing IODP site U1516 in this study and reference sites from previous studies (Dickson et al., 2017; Du Vivier et al., 2015; Pogge von Strandmann et al., 2013); b) Modern regional context of the Mentelle Basin, Naturaliste Plateau, and southwest Australia showing the location of Site U1516 and the potential sediment sources of southwest Australia. E, Eastbourne, UK; WIS, Western Interior Basin; 1260, Ocean Drilling Program (ODP) Site 1260, Demerara Rise; 530, Deep Sea Drilling Project Site 530, Angola Basin; 1138, ODP Site 1138, Kerguelen Plateau. The High Arctic, Caribbean, Ontong-Java, and Madagascar LIPs are marked as HALIP, CLIP, OJP and M, respectively. Modified from Du Vivier et al. (2015).</figcaption></figure>

154

## 2. Materials

156 Site U1516 (34° 20.92'S, 112° 47.96'E; water depth ~2675 m) was drilled in the
157 Mentelle Basin off southwest Australia during the IODP Expedition 369 (Fig. 1),
158 recovering a sedimentary record from the Albian to Pleistocene (Huber et al., 2019).
159 In this study, we mainly focus on the interval between 473.94 and 460.13 m revised
core composited sea floor (rCCSF), which corresponds to the late Cenomanian to early Turonian interval. At Site U1516, the OAE 2 interval was identified between 473.94 and 466.30 rCCSF based on both stratigraphic correlation of the U1516 δ¹³C record with the reference section at Eastbourne (UK) and nannofossil-foraminiferal biostratigraphy (Petrizzo et al., 2021; Tsikos et al., 2004). Note that the stratigraphic position of the OAE 2 interval in this study has been slightly revised from Petrizzo et al. (2021), in order to take into account the presence of a major carbonate dissolution event (e.g., Chen et al., 2022) and the possible diachrony of nannofossil datums at Site U1516 (e.g., Boulila et al., 2020). Yet, additional work based on astronomical tuning will be required to further refine the age model around the OAE 2 interval (Batenburg et al., 2021). The estimated duration of OAE 2 at Site U1516 is ~800 kyr based on the age-depth model established by Petrizzo et al. (2021), which is in agreement with estimates for other stratigraphic sections at high southern latitudes (e.g., Gangl et al., 2019). At Site U1516, the OAE 2 is characterized by the occurrence of a carbonate-poor interval between 467.37 and 470.12 m rCCSF, accompanied by the deposition of a black (organic-rich) shale unit at ~470 m rCCSF (Petrizzo et al., 2021). The rest of the OAE 2 interval is composed of claystone, clayey nannofossil chalk, and calcareous/nannofossil chalk with clay interbedded with chert (Huber et al., 2019). Thirty-five samples from Site U1516 were analyzed in this study. The OAE 2 interval at Site U1516 was split into two main sediment intervals based on sediment provenance, which correspond to the so-called Phase I (between 473.94 and 471.40 m rCCSF) and Phase II (471.40–466.30 m rCCSF) (Fig. 3; Chen et al., 2022). Phase II
corresponds to the most prominent CIE associated with a major provenance change inferred from Nd isotopes, interpreted as reflecting enhanced hydrological cycle in southwest Australia (Chen et al., 2022). Note that the Plenus Cold Event (PCE); an episode of transient cooling during OAE 2 previously identified at several Northern Hemisphere sites (e.g., O'Connor et al., 2020), possibly correlates to the interval between 472.54 and 470.72 m rCCSF at Site U1516, although direct proxy evidence for significant cooling during PCE is yet to be found (Petrizzo et al., 2021).

In order to constrain the potential influence of post-depositional alteration and clay authigenesis (i.e., reverse weathering) on measured Hf isotopic compositions, a suite of sediment samples (n=20) was analyzed from various ODP and IODP sites (n=10) at ocean margins worldwide (Angola margin, Congo Fan, Peru Trench, Chile margin, Hydrate Ridge, Bering Sea, and Papua New Guinea margin). These sites are characterized by high inputs of lithogenic and organic matter, experiencing intense early diagenetic processes that result in methanogenesis, marine silicate weathering under anoxic conditions, and presumably clay authigenesis (Table S1). For each site, sediment samples associated with both low and high total alkalinity (TA) values in pore waters (up to 160 mM; obtained from shipboard data; see Supplementary Information Table S1 and references therein) were analyzed for comparison. Note that high total alkalinity values (TA > ~40 mM) in pore waters are generally taken as indicative of marine silicate weathering (Wallmann et al., 2008).
## 3. Methods

Both Hf and Nd isotopic compositions were conducted on clay-size siliciclastic fractions. Prior to analyses, carbonate, Fe-Mn oxyhydroxide phases and organic matter were removed from the bulk sediment using a sequential leaching procedure, followed by separation of clay-size (<2 µm) detrital fractions using low-speed centrifugation (Bayon et al., 2015; Bayon et al., 2022). Note that all Nd isotope data for the same Site U1516 samples used in this study have been reported in a companion paper focused on sediment provenance (Chen et al., 2022), in which all details on sample preparation and digestion can be found.

The abundances of selected major and trace elements in clay-size fractions were determined at the Pôle Spectrométrie Océan (Brest) using a Thermo Scientific Element XR sector field ICP-MS. The precision for major and trace elements was better than 3% and 5%, respectively. The accuracy of our data was assessed by analyzing five geological-certified reference materials (BHVO-2, AGV-1, BCR-1, DNC-1, and UB-N). The results obtained for the reference materials gave an accuracy typically better than 8%.

Hafnium and neodymium were purified by ion exchange chromatography (Bayon et al., 2016), and Hf-Nd isotopic measurements were conducted at the Pôle Spectrométrie Océan (Brest) using a Thermo Scientific Neptune multi-collector ICP-MS. Hf and Nd isotopic compositions were determined using sample-standard bracketing, by analyzing AMES and Nd Spex standard solutions every two samples,
respectively. All measured Hf and Nd isotopic ratios were corrected from mass bias using ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7129, respectively. During analytical sessions, Hf and Nd isotopic compositions of JMC-475 and JNdi-1 solutions yielded 0.282162 ± 0.000004 (2 SD; n=12) and 0.512116 ± 0.000005 (2 SD; n=8) values similar to reference values. Hf and Nd isotopic ratios are expressed using the epsilon notation, with εHf = [(¹⁷⁶Hf/¹⁷⁷Hf)sample / (¹⁷⁶Hf/¹⁷⁷Hf)CHUR - 1] × 10000 and εNd = [(¹⁴³Nd/¹⁴⁴Nd)sample / (¹⁴³Nd/¹⁴⁴Nd)CHUR - 1] × 10000; using Chondritic Uniform Reservoir (CHUR) values of 0.282785 and 0.512630 for Hf and Nd isotopic ratios, respectively (Bouvier et al., 2008). Initial εNd(i) and εHf(i) values were calculated using elemental Sm/Nd and Lu/Hf ratios (see details in Bayon et al., 2022) and stratigraphic ages provided by the age model (Petrizzo et al., 2021).

237

## 4. Results

### 4.1 Nd-Hf isotopic compositions of clay-size fractions at Site U1516

Hafnium isotope ratios for clay-size siliciclastic fractions at Site U1516 are reported in Table 1, together with recently published Nd isotope data for the same samples (Chen et al., 2022). Corresponding major and trace element data are listed in the supplementary dataset.

Note that three samples of clay-size fraction exhibiting anomalous shale-normalized rare earth element (REE) patterns, indicating the presence of residual authigenic phases or accessory heavy minerals, were discarded for the discussion based on Nd-Hf isotopes (see Supplementary text; Fig. S1). All clay-size
fractions at Site U1516 show negative εHf(i) values, ranging between -18.2 and -7.7 (Table 1), and plot between the correlation trends defined by the Clay Array and the Terrestrial Array (Fig. 2). The vertical deviation of measured Hf isotopic compositions relative to the Clay Array yields ΔεHfclay values ranging between -11.5 and +1.8 (Table 1 and Fig. 2). Unlike Nd isotopes, which displayed a shift towards unradiogenic values (from -11.1 to 23.2) during Phase II of the OAE 2 (Fig. 2) (Chen et al., 2022), εHf(i) values exhibit relatively smaller variability across the OAE 2 (between -14.9 and -7.7), except for a prominent shift towards highly negative values (down to ~ -18.2) within the black shale unit (Fig. 2). The degree of Hf-Nd isotope decoupling in clay-size fractions (ΔεHfclay) displays two major changes at Site U1516: a first shift from ~-5.9 to ~-11.5 at the beginning of Phase II of the OAE 2, with the lowest values coincident with black shale deposition, followed by a major shift towards positive ΔεHfclay values between 470.26 and 468.84 m rCCSF (Fig. 2).

261

262 4.2 Hf and Nd isotopes in methanogenic sediments

263 Clay-size fractions separated from methanogenic sediments experiencing marine 264 silicate weathering at ocean margins display a large range of Hf and Nd isotopic 265 compositions (Fig. 2 and Table S1), from unradiogenic values typical of passive 266 continental margin settings (Black Ridge, Angola margin and Congo Fan) to 267 radiogenic values at active ocean margins (Peru trench, Chile margin, Hydrate Ridge, 268 Bering Sea and Papua New Guinea margin). At each site, clay-size fractions extracted 269 from sediment layers associated with both low and high TA values in pore waters
display similar $\epsilon_{Nd(i)}$ and $\epsilon_{Hf(i)}$ values (Fig. 2 and Table S1), plotting near the Clay Array and the field of volcanogenic river clays (Bayon et al., 2016), thus yielding near-zero $\Delta\epsilon_{Hfclay}$ values.

<figure><img src="image_4.png" /></figure>

**Fig. 2.** Hf and Nd isotopic compositions of clay-size fractions of Site U1516 (red circles) and other continental margin sediments recovered from low- and high-alkalinity pore-water environments. Data for modern river clays and silts worldwide are shown for comparison (Bayon et al., 2016; Bayon et al., 2022), together with the present-day Terrestrial Array ($\epsilon_{Hf} = 1.55 \times \epsilon_{Nd} + 1.21$; dark grey line; Vervoort et al., 2011), the Clay Array ($\epsilon_{Hf} = 0.78 \times \epsilon_{Nd} + 5.23$; red line; Bayon et al., 2016), and the Seawater Array ($\epsilon_{Hf} = 0.55 \times \epsilon_{Nd} + 7.1$; blue line; Albarède et al., 1998). Green squares refer to clay-size fractions separated from continental margin sediments associated with pore waters displaying low-alkalinity (seawater-like) values, while blue squares correspond to samples with high-alkalinity pore water levels, indicative of anoxic silicate weathering.

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<table><caption>**Table 1** Hf and Nd isotopic composition of the clay-size fraction of OAE 2 sediments at IODP Site U1516.</caption><thead><tr><th>Sample ID</th><th>Hole</th><th>Core</th><th>Type</th><th>Section</th><th>Top offset (cm)</th><th>Bottom offset (cm)</th><th>rCCSF (m)</th><th><sup>176</sup>Hf/<sup>177</sup>Hf ± 2 se</th><th><sup>143</sup>Nd/<sup>144</sup>Nd ± 2 se*</th><th>ε<sub>Hf(i)</sub></th><th>ε<sub>Nd(i)</sub></th><th>Δε<sub>Hfclay</sub></th></tr></thead><tbody><tr><td>M1</td><td>D</td><td>2</td><td>R</td><td>3</td><td>8</td><td>9</td><td>460.28</td><td>0.282442 ± 9</td><td>0.5118026 ± 4</td><td>-10.6</td><td>-15.2</td><td>-4.0</td></tr><tr><td>M2</td><td>C</td><td>30</td><td>R</td><td>2</td><td>50</td><td>51</td><td>461.22</td><td>0.282342 ± 9</td><td>0.5119581 ± 3</td><td>-14.6</td><td>-12.1</td><td>-10.4</td></tr><tr><td>M3</td><td>D</td><td>3</td><td>R</td><td>2</td><td>71</td><td>72</td><td>464.96</td><td>0.282437 ± 9</td><td>0.5118602 ± 4</td><td>-11.3</td><td>-13.9</td><td>-5.6</td></tr><tr><td>M4</td><td>C</td><td>31</td><td>R</td><td>2</td><td>27</td><td>28</td><td>465.99</td><td>0.282355 ± 8</td><td>0.5119102 ± 3</td><td>-14.7</td><td>-13.0</td><td>-9.8</td></tr><tr><td>M5</td><td>C</td><td>31</td><td>R</td><td>2</td><td>60</td><td>61</td><td>466.32</td><td>0.282483 ± 9</td><td>0.5119989 ± 2</td><td>-11.8</td><td>-11.4</td><td>-8.2</td></tr><tr><td>M6</td><td>C</td><td>31</td><td>R</td><td>2</td><td>95</td><td>96</td><td>466.67</td><td>0.282394 ± 7</td><td>0.5118602 ± 3</td><td>-13.4</td><td>-14.3</td><td>-7.5</td></tr><tr><td>M7</td><td>C</td><td>31</td><td>R</td><td>3</td><td>35</td><td>36</td><td>467.34</td><td>0.282444 ± 7</td><td>0.5118125 ± 3</td><td>-11.6</td><td>-15.0</td><td>-5.1</td></tr><tr><td>M8</td><td>D</td><td>4</td><td>R</td><td>1</td><td>95</td><td>97</td><td>467.95</td><td>0.282388 ± 7</td><td>0.5117960 ± 4</td><td>-13.3</td><td>-15.2</td><td>-6.6</td></tr><tr><td>M9</td><td>D</td><td>4</td><td>R</td><td>1</td><td>125</td><td>127</td><td>468.25</td><td>0.282398 ± 8</td><td>0.5115885 ± 5</td><td>-12.5</td><td>-19.2</td><td>-2.8</td></tr><tr><td>M10</td><td>D</td><td>4</td><td>R</td><td>2</td><td>22</td><td>24</td><td>468.51</td><td>0.282397 ± 7</td><td>0.5114325 ± 6</td><td>-12.6</td><td>-22.3</td><td>-0.4</td></tr><tr><td>M11</td><td>D</td><td>4</td><td>R</td><td>2</td><td>55</td><td>57</td><td>468.84</td><td>0.282489 ± 8</td><td>0.5115904 ± 3</td><td>-9.7</td><td>-19.2</td><td>0.1</td></tr><tr><td>M12</td><td>D</td><td>4</td><td>R</td><td>2</td><td>86</td><td>88</td><td>469.15</td><td>0.282507 ± 6</td><td>0.5117217 ± 4</td><td>-9.1</td><td>-16.7</td><td>-1.2</td></tr><tr><td>M13</td><td>D</td><td>4</td><td>R</td><td>3</td><td>1</td><td>3</td><td>469.44</td><td>0.282495 ± 7</td><td>0.5115217 ± 4</td><td>-9.0</td><td>-20.5</td><td>1.7</td></tr><tr><td>M14</td><td>D</td><td>4</td><td>R</td><td>3</td><td>15</td><td>16</td><td>469.57</td><td>0.282435 ± 6</td><td>0.5113823 ± 4</td><td>-11.0</td><td>-23.2</td><td>1.8</td></tr><tr><td>M15</td><td>D</td><td>4</td><td>R</td><td>3</td><td>47</td><td>49</td><td>469.90</td><td>0.282407 ± 7</td><td>0.5117544 ± 4</td><td>-12.2</td><td>-15.9</td><td>-5.1</td></tr><tr><td>M16</td><td>D</td><td>4</td><td>R</td><td>3</td><td>51</td><td>52</td><td>469.94</td><td>0.282399 ± 6</td><td>0.5116689 ± 3</td><td>-12.6</td><td>-17.6</td><td>-4.1</td></tr><tr><td>M17</td><td>D</td><td>4</td><td>R</td><td>3</td><td>55</td><td>56</td><td>469.98</td><td>0.282453 ± 8</td><td>0.5117427 ± 4</td><td>-10.9</td><td>-16.1</td><td>-3.6</td></tr><tr><td>M18</td><td>D</td><td>4</td><td>R</td><td>3</td><td>59</td><td>60</td><td>470.02</td><td>0.282407 ± 11</td><td>0.5116167 ± 2</td><td>-12.1</td><td>-18.7</td><td>-2.7</td></tr><tr><td>M19</td><td>D</td><td>4</td><td>R</td><td>3</td><td>62</td><td>63</td><td>470.05</td><td>0.282438 ± 9</td><td>0.5116524 ± 4</td><td>-10.8</td><td>-17.9</td><td>-2.0</td></tr><tr><td>M20</td><td>D</td><td>4</td><td>R</td><td>3</td><td>66</td><td>68</td><td>470.09</td><td>0.282363 ± 12</td><td>0.5117652 ± 4</td><td>-14.0</td><td>-15.8</td><td>-6.9</td></tr><tr><td>M21</td><td>D</td><td>4</td><td>R</td><td>3</td><td>76</td><td>78</td><td>470.19</td><td>0.282332 ± 8</td><td>0.5117594 ± 5</td><td>-14.9</td><td>-15.9</td><td>-7.7</td></tr><tr><td>M22</td><td>D</td><td>4</td><td>R</td><td>3</td><td>83</td><td>85</td><td>470.26</td><td>0.282255 ± 13</td><td>0.5118258 ± 4</td><td>-17.8</td><td>-14.7</td><td>-11.5</td></tr><tr><td>M23</td><td>D</td><td>4</td><td>R</td><td>3</td><td>93</td><td>95</td><td>470.36</td><td>0.282342 ± 7</td><td>0.5118490 ± 4</td><td>-14.5</td><td>-14.2</td><td>-8.6</td></tr><tr><td>M24</td><td>D</td><td>4</td><td>R</td><td>3</td><td>96</td><td>97</td><td>470.39</td><td>0.282379 ± 10</td><td>0.5118529 ± 3</td><td>-13.2</td><td>-14.1</td><td>-7.4</td></tr></tbody></table>
<table>
  <tr>
    <td>M25</td>
    <td>D 4</td>
    <td>R</td>
    <td>3</td>
    <td>104</td>
    <td>106</td>
    <td>470.47</td>
    <td>0.282230 ± 8</td>
    <td>0.5117439 ± 5</td>
    <td>-18.2</td>
    <td>-16.1</td>
    <td>-10.9</td>
  </tr>
  <tr>
    <td>M26</td>
    <td>D 4</td>
    <td>R</td>
    <td>3</td>
    <td>124</td>
    <td>126</td>
    <td>470.67</td>
    <td>0.282414 ± 11</td>
    <td>0.5119031 ± 6</td>
    <td>-12.1</td>
    <td>-13.2</td>
    <td>-7.1</td>
  </tr>
  <tr>
    <td>M27</td>
    <td>D 4</td>
    <td>R</td>
    <td>3</td>
    <td>135</td>
    <td>136</td>
    <td>470.77</td>
    <td>0.282416 ± 15</td>
    <td>0.5118824 ± 5</td>
    <td>-12.3</td>
    <td>-13.8</td>
    <td>-6.8</td>
  </tr>
  <tr>
    <td>M28</td>
    <td>D 4</td>
    <td>R</td>
    <td>4</td>
    <td>1</td>
    <td>3</td>
    <td>470.94</td>
    <td>0.282470 ± 9</td>
    <td>0.5119418 ± 6</td>
    <td>-10.9</td>
    <td>-12.6</td>
    <td>-6.2</td>
  </tr>
  <tr>
    <td>M29</td>
    <td>D 4</td>
    <td>R</td>
    <td>4</td>
    <td>13</td>
    <td>15</td>
    <td>471.06</td>
    <td>0.282495 ± 14</td>
    <td>0.5120234 ± 4</td>
    <td>-14.4</td>
    <td>-11.0</td>
    <td>-11.0</td>
  </tr>
  <tr>
    <td>M30</td>
    <td>D 4</td>
    <td>R</td>
    <td>4</td>
    <td>37</td>
    <td>39</td>
    <td>471.30</td>
    <td>0.282425 ± 6</td>
    <td>0.5120126 ± 6</td>
    <td>-12.1</td>
    <td>-11.1</td>
    <td>-8.7</td>
  </tr>
  <tr>
    <td>M31</td>
    <td>D 4</td>
    <td>R</td>
    <td>4</td>
    <td>61</td>
    <td>63</td>
    <td>471.54</td>
    <td>0.282547 ± 25</td>
    <td>0.5119490 ± 7</td>
    <td>-7.7</td>
    <td>-12.4</td>
    <td>-3.2</td>
  </tr>
  <tr>
    <td>M32</td>
    <td>D 4</td>
    <td>R</td>
    <td>4</td>
    <td>96</td>
    <td>98</td>
    <td>471.90</td>
    <td>0.282540 ± 6</td>
    <td>0.5120229 ± 4</td>
    <td>-8.1</td>
    <td>-11.0</td>
    <td>-4.8</td>
  </tr>
  <tr>
    <td>M33</td>
    <td>D 4</td>
    <td>R</td>
    <td>5</td>
    <td>28</td>
    <td>30</td>
    <td>472.38</td>
    <td>0.282473 ± 16</td>
    <td>0.5119455 ± 3</td>
    <td>-12.5</td>
    <td>-12.5</td>
    <td>-8.0</td>
  </tr>
  <tr>
    <td>M34</td>
    <td>C 32</td>
    <td>R</td>
    <td>2</td>
    <td>140</td>
    <td>141</td>
    <td>472.85</td>
    <td>0.282480 ± 6</td>
    <td>0.5119108 ± 5</td>
    <td>-11.3</td>
    <td>-13.1</td>
    <td>-6.2</td>
  </tr>
  <tr>
    <td>M35</td>
    <td>C 32</td>
    <td>R</td>
    <td>3</td>
    <td>60</td>
    <td>61</td>
    <td>473.56</td>
    <td>0.282484 ± 15</td>
    <td>0.5118433 ± 3</td>
    <td>-10.4</td>
    <td>-14.4</td>
    <td>-4.4</td>
  </tr>
</table>
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**5. Discussion**

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**5.1 Behaviour of Hf isotopes during reverse weathering**

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A prerequisite for the use of $\Delta\varepsilon_{Hfclay}$ as a paleoweathering proxy is that the degree of Hf-Nd isotope decoupling in clay-size sediments is not controlled by the presence of unradiogenic zircon nor by any post-depositional processes. While the presence of zircon in clay-size fractions can be assessed using Zr and shale-normalized heavy-REE abundances (see section 5.2), to date, there is little information about the behaviour of Nd and Hf isotopes during diagenesis, especially during *in situ* formation of aluminosilicate minerals, a process referred to as 'reverse weathering' in marine sediments (Michalopoulos and Aller, 1995). During reverse weathering, the combined dissolution of biogenic opal, metal hydroxides and organic matter under anoxic conditions leads to rapid precipitation of authigenic clay minerals (Michalopoulos and Aller, 1995; Wallmann et al., 2008). This process typically results in the preferential incorporation and/or absorption of dissolved cations (e.g., Fe, K, and Mg) from surrounding pore waters into newly formed clay minerals, associated with kinetic isotope effects (e.g., Santiago Ramos et al., 2018). Reverse weathering generally occurs in close association with the dissolution of detrital silicate minerals under anoxic conditions; a process referred to as marine silicate weathering, which can be inferred from anomalously high total alkalinity (TA) levels (>40 mM) in continental margin sediments (Wallmann et al., 2008), or from radiogenic Sr isotopic compositions of pore waters departing from seawater values (e.g., Torres et al., 2020). At Site U1516, present-day alkalinity levels in interstitial waters never exceed ~6.5 mM in the upper, carbonate-dominated, sedimentary column (<440 mbsf), reflecting limited organic matter remineralization rate (Huber et al., 2019). However, during
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OAE 2, rapid accumulation of abundant terrigenous material, biogenic opal and organic matter at the seafloor, combined with the onset of anoxic deep-water conditions (Petrizzo et al., 2021; Chen et al., 2022), provided ideal conditions for post-depositional dissolution of silicate minerals coupled with clay authigenesis (reverse weathering). Such intense early diagenetic processes could have led to strong Hf-Nd isotope decoupling in corresponding clay-size fractions, thereby biasing the interpretation of ΔεHfclay as a paleoproxy for continental chemical weathering.

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In this study, the influence of marine silicate weathering and subsequent clay authigenesis on Nd-Hf isotope decoupling has been evaluated using the suite of samples collected from methanogenic sediment layers at continental margins. At present, these sediment samples probably experience the same diagenetic conditions as the sediment deposited at Site U1516 during OAE 2. While both Nd and Hf isotopic compositions of studied clay-size fractions vary from one site to another, reflecting different geological settings and provenance, clay-size fractions at each site display similar ΔεHfclay values regardless of corresponding TA levels in pore waters (Fig. 2 and Table S1). This observation suggests no measurable influence of reverse weathering on the Hf-Nd isotope compositions of clay-size fractions, despite clear evidence for ongoing marine silicate weathering (TA >40 mM) at all studied sites (Fig. 2 and Table S1). To some extent, the lack of any particular ΔεHfclay difference between clays recovered from sediment layers having experienced differing degrees of marine silicate and reverse weathering intensity reflects the fact that both Hf and REE are present in very low concentrations in pore waters relative to detrital silicate minerals. In agreement with recent

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findings suggesting that the REE cycling during clay marine authigenesis is coupled with dissolution of detrital clay minerals (Abbott et al., 2019), our results suggest that marine silicate weathering under anoxic conditions is possibly followed by quantitative incorporation of released REE and Hf into neoformed authigenic clay minerals during reverse weathering, hence preserving inherited detrital Nd and Hf isotopic signatures. In this context, our new data provide empirical support for the reliability of Hf-Nd isotopes as continental weathering proxies in organic-rich marine sediments experiencing reverse weathering. By analogy, therefore, we infer that during OAE 2, the deposition of organic-rich black shale sediment at Site U1516, presumably associated with the co-occurrence of marine silicate weathering and clay authigenesis in the sub-seafloor environment, had negligible impact on the ΔεHfclay composition of clay-size detrital minerals.

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## 5.2 Application of Hf isotopes as paleoweathering proxies

In this study, the evolution of continental chemical weathering during OAE 2 is reconstructed using ΔεHfclay (Bayon et al., 2016) (Fig. 3). The use of this quantitative measure of Hf-Nd isotope decoupling as a weathering proxy is based on the evidence that ΔεHfclay in modern river clays displays strong relationships with climatic parameters of corresponding river basins (Bayon et al., 2016). Positive ΔεHfclay values indicate more intense chemical weathering under warm and wet climate conditions, reflecting presumably the recycling of radiogenic apatite and other secondary phosphate-bearing minerals in soils (Banfield and Eggleton, 1989; Bayon et al., 2016). In soils, preferential dissolution of poorly resistant accessory minerals such as allanite and apatite typically leads to formation of secondary
phosphate minerals such as florencite and rhabdophane (Banfield and Eggleton, 1989). While being more stable than their corresponding primary minerals, these secondary phases can undergo dissolution under more intense weathering conditions (Banfield and Eggleton, 1989), presumably releasing a substantial fraction of radiogenic Hf in soils that can be subsequently incorporated into neoformed clay minerals. In contrast, the most negative ΔεHfclay values in river-borne sediments are typically encountered in subarctic catchments draining crystalline rocks that undergo limited chemical weathering, reflecting the overwhelming presence of primary rock-forming minerals relative to secondary clay mineral assemblages (Bindemann et al., 2019). Nevertheless, particular care shall be taken prior to using ΔεHfclay as a weathering proxy, because additional factors such as mineral sorting, provenance, or sediment recycling can also potentially influence the Hf isotopic composition of fine-grained siliciclastic sediments (Bayon et al., 2016). Below, we briefly review these potential factors, in order to show that they are unlikely to affect the use of ΔεHfclay as a paleoweathering proxy at Site U1516.

First, the relatively low Zr abundances (average 67.5 ± 12 µg/g) in clay-size fractions at Site U1516 and the absence of correlation trend between Zr/Rb ratios and εHf values (R² = 0.08; plot not shown here) both indicate negligible influence of a putative zircon effect. Except for three samples that were discarded (see Supplementary text), the absence of characteristic heavy-REE enrichments in shale-normalized REE patterns also points towards the absence of zircon or any other heavy accessory minerals in studied clay-size fractions (Fig. S1).
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In addition to the zircon effect, the presence of mafic rocks in the source regions, which weather faster than other types of rocks, can yield much higher ΔεHfclay in fine-grained volcanogenic sediments transported by river systems (Bayon et al., 2016; Bayon et al., 2022). In this study, the potential influence of basalt weathering on measured ΔεHfclay values at Site U1516 can be excluded due to insignificant contribution of volcanogenic material from the Kerguelen Plateau during OAE 2 (Chen et al., 2022).

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Recycling of old sedimentary material derived from former erosional cycles could represent an additional factor complicating the use of ΔεHfclay or any other weathering proxy in the sedimentary record. However, as discussed in previous studies, the close relationship commonly observed in catchments between modern climatic parameters and the degree of weathering of fine-grained sediments derived from ancient shale formations suggests that the material exported from river systems, at least at passive margins, can still carry useful information on contemporaneous weathering conditions (e.g., Bayon et al., 2018; Dere et al., 2013). Additionally, clay-size detrital sediments at Site U1516 were mainly transported from drainage basins dominated by Precambrian granitoids and gneisses in southwest Australia (e.g., Chen et al., 2022 and references therein), hence further precluding any significant impact of sedimentary recycling in the interpretation of our weathering proxy records.

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Concerning the potential influence of sediment provenance, its evolution at Site U1516 across OAE 2 has been recently discussed in detail (Chen et al., 2022), indicating a major change during Phase II of the OAE 2, coinciding with a shift towards more negative εNd values. Chen et al. (2022) interpreted this major εNd shift as reflecting major drainage reorganization in southwest Australia, associated with enhanced contribution from distant

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sediment sources (Yilgarn Craton and Albany-Fraser Orogen) relative to proximal regions (Leeuwin Block and Perth Basin) (Figs. 1 and 3). However, in this study, the absence of any significant correlation between $\varepsilon_{Hf(i)}$ and $\varepsilon_{Nd(i)}$ in clay-size fractions ($R^2 = 0.01, p < 0.05$), nor of any relationships between median grain size and $\varepsilon_{Hf(i)}$ ($R^2 = 0.02, p < 0.05$), precludes any major control of sediment provenance on measured $\Delta\varepsilon_{Hfclay}$ values at Site U1516 (Fig. 3).

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Therefore, except for a particular sediment interval that will be specifically discussed below (see section 5.3), the observed $\Delta\varepsilon_{Hfclay}$ variability at Site U1516 is probably best explained by changes in chemical weathering intensity, as proposed in previous studies (Bayon et al., 2016; Bayon et al., 2022; Corentin et al., 2022). Interestingly, as illustrated in Fig. 3, conventional indices of chemical weathering such as the chemical index of weathering (CIA) and potassium/aluminum (K/Al) ratios, which are commonly used to track the intensity of feldspar weathering, display no correlation with $\Delta\varepsilon_{Hfclay}$ during Phase II of the OAE 2. In contrast to $\Delta\varepsilon_{Hfclay}$, we infer that the observed shifts in CIA and K/Al at Site U1516 are mostly caused by changes in sediment provenance associated with enhanced delivery of K-bearing mineral assemblages such as illite during OAE 2 (Chen et al., 2022). This assumption is supported by the strong relationship observed between illite contents and K/Al ($R^2 = 0.47, p < 0.05$), and evidence that both K/Al and CIA display significant correlation with Nd isotope ratios at Site U1516 ($R^2_{K/Al \text{ vs. } \varepsilon_{Nd}} = 0.42, R^2_{CIA \text{ vs. } \varepsilon_{Nd}} = 0.25, p < 0.05$) (Fig. 3).

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To summarize, none of the potential additional controls on $\Delta\varepsilon_{Hfclay}$ at Site U1516, namely the zircon effect, the presence of volcanogenic sediment, sediment recycling, or provenance

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change, are likely to explain the observed ΔεHfclay variability at Site U1516 during OAE 2.

Therefore, on the basis of the relationships previously documented between ΔεHfclay and the degree of chemical weathering intensity in modern river clays, we propose that the ΔεHfclay shift towards positive values identified at Site U1516 was induced by intensifying chemical weathering in the source region (southwest Australia).

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Stratigraphy, lithology and geochemistry of Site U1516 sediments during the OAE 2 interval. a) δ¹³C and b) δ¹⁸O of bulk carbonate (Petrizzo et al., 2021); c) total organic carbon (TOC) contents (Chen et al., 2022); d) εNd of clay-size fractions and light-REE/heavy-REE (LREE/HREE) ratio; e) εHf of clay-size fractions; f) ΔεHfclay; g) K/Al ratios (in reverse axis); h) CIA; and i) illite content. The light green and grey bands indicate the OAE 2 interval and black shale unit, respectively.</figcaption></figure>

5.3 Sediment flushing following abrupt river reactivation at the onset of Phase II of OAE 2
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A striking feature of our results is the abrupt ΔεHf(i)clay excursion towards highly negative values (-11.5) at the onset of Phase II of the OAE 2, with values even departing from the observed ΔεHfclay range in modern river sediments (Fig. 4c). As discussed above, neither reverse weathering nor the presence of unradiogenic zircons is likely to explain this shift towards highly negative ΔεHfclay values at Site U1516. Very low ΔεHfclay values in modern river systems are typically encountered in cold and dry environments dominated by physical erosion, where clay-size fractions are mostly composed of primary mineral assemblages (Bayon et al., 2016; Bindeman et al., 2019). By analogy, the highly negative ΔεHfclay excursion at Site U1516 could hence indicate the presence of subarctic conditions at that time, associated with limited chemical weathering. However, this hypothesis is incompatible with evidence for relatively high atmospheric temperature (>20 °C) and the onset of hydrological cycle during this particular time interval (Chen et al., 2022; Petrizzo et al., 2021). Evidence for global expansion of forests from middle-to-high latitudes during the mid-Cretaceous also precludes this hypothesis (Zhou et al., 2012). Alternatively, more congruent silicate weathering favoring the alteration of highly-resistant zircons could have possibly led to the release of highly unradiogenic Hf to surface environments, subsequently incorporated by secondary clay minerals upon formation in soils. Most likely, the near-absence of kaolinite at Site U1516, a clay mineral typical of very intense chemical weathering, rules out this hypothesis. Instead, we propose that the ΔεHfclay shift towards highly negative values at Site U1516 indicates the presence of unweathered primary mineral assemblages rapidly flushed from nearby southwest Australia following the reactivation of river systems. At Site U1516, this particular episode immediately post-dates the late stage of Phase I (Fig. 4), characterized
by relatively high εNd(i) and Zr/Rb values, indicative of reduced riverine discharge (and
hydrological cycle) and enhanced eolian dust deposition, hence suggestive of arid conditions
(Chen et al., 2022). At the onset of Phase II of the OAE 2, the pronounced εNd(i) shift towards
unradiogenic values indicates markedly increasing contribution of terrigenous sediments from
remote regions in southwest Australia (e.g., Yilgarn Craton; Fig. 1; Chen et al., 2022). This
major shift was previously interpreted as reflecting the reactivation of large river systems
induced by enhanced hydrological cycle during OAE 2 (Fig. 4; Chen et al., 2022). At that
time, the onset of heavy rainfall episodes could have remobilized poorly weathered sediments
previously accumulated in inactive drainage systems, possibly from older soil horizons
developed under subarctic conditions, or resulted in rapid erosion of unvegetated bare rock
surfaces with limited chemical weathering. To some extent, such a scenario echoes with the
abrupt reactivation of paleodrainage systems previously documented in arid North Africa
during the Quaternary, which repeatedly occurred following short-term latitudinal shifts of
monsoonal rainfall (e.g., Blanchet et al., 2021; Skonieczny et al., 2015). During these humid
intervals, moderate annual precipitation levels in North Africa (~ <700 mm/yr) were sufficient
to trigger markedly increased fluvial discharges at nearby continental margins (Blanchet et al.,
2021). In southwest Australia, such flushing flows leading to remobilization of riverbank
sediments or ancient soil horizons during OAE 2 could explain the overwhelming presence of
primary mineral assemblages in the exported fine-grained sediment load deposited at Site
U1516, as inferred from the ΔεHfclay excursion towards highly negative values (Fig. 4).

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5.4 Enhanced weatherability triggered by southward migration of Southern Hemisphere westerlies and global implications

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During the OAE 2, the rise of temperature caused by massive emission of greenhouse gases following the emplacement of LIPs and resulting intensification of the global water cycle is thought to have led to globally enhanced chemical weathering (e.g., Jenkyns, 2010), due to the dependency of silicate mineral dissolution rates to temperature and precipitation. At Site U1516, the major Δε<sub>Hfclay</sub> increase from -11.5 to +1.8 during this period is fully consistent with intensifying chemical weathering conditions (Fig. 4). Bulk carbonate δ<sup>18</sup>O indicates that sea-surface paleotemperature (SST) at Site U1516 peaked during OAE 2 and remained relatively stable between ~20–23.5°C (Petrizzo et al., 2021), consistent with another SST record at Southern Hemisphere mid-latitudes (ODP Site 1138; Fig. 1; paleolatitude ~47°S) (Robinson et al., 2019). In southwest Australia, the intensification of chemical weathering inferred from Δε<sub>Hfclay</sub> coincides with the most prominent CIE at Site U1516, but also with a major ε<sub>Nd(i)</sub> shift in clay-size fractions towards unradiogenic values (Fig. 4). As mentioned above, this latter ε<sub>Nd(i)</sub> shift has been interpreted as reflecting the onset of intense hydrological cycle in southwest Australia, associated with riverine export of terrigenous material from distant source regions (Fig. 4b). The strong relationship between Δε<sub>Hfclay</sub> and ε<sub>Nd(i)</sub> during this particular period of the OAE 2 (R<sup>2</sup> = 0.57 during Phase II, *p* <0.05) suggests that the intensification of chemical weathering was caused by increasing precipitation levels rather than by rising atmospheric temperature changes. By comparing our results to those for modern river clays (Bayon et al., 2016), we infer that the Δε<sub>Hfclay</sub> shift towards positive values during phase II of OAE 2 indicates a transition from reduced chemical weathering under arid
climate to moderate weathering intensity associated with humid conditions. This observation
is coherent with recent paleontological investigations suggesting the development of
temperate-warm and humid climate at high latitudes during the mid-Cretaceous (Varela et al.,
2018).

Taken together, our combined evidence for enhanced fluvial discharges (inferred from Nd
isotopes and clay mineralogy; Chen et al., 2022) and intensifying chemical weathering
(inferred from ΔεHfclay; this study) hence provide direct support for a greater susceptibility of
the land surface to chemical erosion (i.e., weatherability) during OAE 2 (Fig. 4). This finding
of a greater weatherability at the regional scale of southwest Australia during OAE 2 is
consistent with existing δ⁷Li and ⁸⁷Sr/⁸⁶Sr proxy record of global seawater chemistry, which
collectively point towards enhanced continental fluxes to the ocean at this period (Pogge von
Strandmann et al., 2013; Yobo et al., 2021), despite some discrepancies in the timing of
proxy-inferred weathering changes that most likely relate to the regional (ΔεHfclay) versus
global (e.g., δ⁷Li) significance of studied proxies (Fig. 4).

By extrapolation, our combined Nd-Hf isotope evidence for enhanced rainfall and
intensifying chemical weathering in southwest Australia implies the possibility that similar
conditions prevailed in other high-latitude regions during OAE 2 (e.g., Gangl et al., 2019).
While the hydroclimate evolution of southern high latitudes during OAE 2 remains poorly
documented, various climate proxy data and modeling studies have suggested the occurrence
of humid conditions at middle-high latitudes during the mid-Cretaceous (Klages et al., 2020;
Varela et al., 2018), possibly related to a drastic shrinking of the Hadley circulation and
poleward migration of the westerlies (e.g., Hasegawa et al., 2012). Our Nd-Hf isotopic data
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provide direct support to the above-mentioned studies, further suggesting that the onset of OAE 2 was associated with enhanced hydrological cycle in high-latitude regions. Although the total areal extent of the emerged continental surface decreased during the mid-Cretaceous, due to global transgression, a substantial fraction of continental landmasses was located in middle-high latitude areas during that period (Fig. 1). By inference, the onset of enhanced weatherability during the peak OAE 2 warmth probably resulted in accelerated nutrient releases to the ocean, promoting high primary productivity at ocean margins surrounding middle-high latitude regions. To summarize, our findings support the view that enhanced weatherability in middle-high latitude regions, associated with the poleward migration of the southern westerlies during the peak OAE 2 warmth, possibly played a key role in atmospheric CO₂ drawdown at that time, through accelerated continental chemical weathering of silicate rocks and organic carbon burial in marine sediments, which altogether could have contributed to the termination of OAE 2. Future studies combining proxy records for chemical weathering at different latitudes and coupled ocean-atmosphere models integrating paleogeographic reconstructions and global carbon cycle will be required to test the reliability of this hypothesis.
<figure><img src="image_6.png" /><figcaption>Fig. 4. a) δ¹³C of bulk carbonate (Petrizzo et al., 2021), b) εNd(i), and c) ΔεHfclay at Site U1516 compared with d) δ⁷Li records of marine carbonate sections in Eastbourne, UK (Pogge von Strandmann et al., 2013). The global range of ΔεHfclay in modern river sediments and for different climatic zones (subarctic, temperate, humid) are also shown for comparison (Bayon et al., 2016). Note that a few outliers for ΔεHfclay in modern river sediments are indicated as red, yellow, and blue dots, respectively. The light green and grey bands indicate OAE 2 intervals and black shale units, respectively. A-A = Australia-Antarctica.</figcaption></figure>

Concluding remarks

The novel ΔεHfclay proxy for chemical weathering was applied to a sedimentary sequence from the Mentelle Basin (Site U1516), retrieved during IODP Expedition 369 in order to reconstruct the evolution of continental weathering at southern high latitudes during the Ocean Anoxic Event (OAE 2). Complementary Hf-Nd isotope analyses of a suite of modern
methanogenic sediments influenced by marine silicate weathering suggest that $\Delta\varepsilon_{Hfclay}$ is
negligibly affected by post-depositional processes such as reverse weathering, and hence can
serve as a reliable proxy for chemical weathering during extreme climate events such as OAE
2. At Site U1516, the evolution of $\Delta\varepsilon_{Hfclay}$ provides first direct evidence for a major shift
towards markedly enhanced weatherability in southwest Australia during the peak OAE 2
warmth, i.e., the interval characterized by the most prominent CIE. We interpret this result as
reflecting the onset of hydrological cycle in southern high latitudes due to the poleward
migration of the westerlies, possibly associated with a drastic shrinking of the Hadley
circulation at that time. These findings emphasize the important role of chemical weathering
in middle-high latitude regions as a potential sink for atmospheric CO$_2$ during OAE 2, which,
together with associated nutrient export to the ocean, primary productivity and organic matter
burial in the sea, possibly allowed the recovery of the Earth's climate system from a
super-greenhouse state.

565

566 Acknowledgment

This research used samples provided by ODP and IODP. This study is financially supported
by the National Key Research and Development Program of China (2022YFF0800503), the
National Natural Science Foundation of China (41876034), the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDB42000000 and XDB40010100), the
Marine S&T Fund of Shandong Province for Pilot National Laboratory for Marine Science
and Technology (Qingdao; 2022QNLM050203), the National Natural Science Foundation of
China (41676038 and 41830539), and Taishan Scholars Project Funding, and a grant to G.B.
574 from the French National Research Agency (ANR-20-CE01-0003). We are most grateful to

575 the Editor (A.D. Jacobson) and two anonymous reviewers for providing insightful and

576 constructive comments. H.C. acknowledges the China Scholarship Council for supporting a

577 research visit at IFREMER. M.-L. Rouget, B. Gueguen and A. Trinquier are thanked for

578 support during elemental and isotopic analyses, respectively.
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