Orbital hydroclimate variability revealed by grain-size
evidence in the tropical Pacific Islands since 140 ka

Xiaojie Tang<fnref n="1" />, Zhaojie Yu<fnref n="2" />, Zhengyao Lu<fnref n="3" />, Christophe Colin<fnref n="1" />

The past evolution of precipitation and atmospheric convection in the Western Pacific Warm Pool (WPWP) is critical for global climate changes but is under debate because of its forcing mechanisms. Here, we present a high temporal resolution (~156 years) grain-size record of core MD01-2385 over the last 140 kyr, in offshore northern New Guinea to reveal sediment dynamics as a proxy for precipitation changes. End-member analysis revealed that a two-endmember model was optimal. The end-member 1/end-member 2 (EM1/EM2) ratio could represent the variation in grain size and exhibited significant precessional cycles changes in phase with modelled Ni no 3 SST anomaly from a global climate model transient simulation. From these data, we inferred orbital fluctuations in precipitation from tropical western Pacific islands, with general precipitation peaks during the time of perihelion at the boreal autumnal equinox (midpoint from a low to high precession index), corresponding to La Ni na-like conditions and vice versa. Comparisons of our new record with published precipitation records showed that orbital precipitation changes in the WPWP are mainly dominated by El Ni no-Southern Oscillation-like (ENSO-like) oscillations in the precession band, while the Intertropical Convergence Zone (ITCZ) mainly controls the distribution of precipitation over a larger spatial area.

<footnote n="1">¹ Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="2">² CAS Institute of Oceanology (IOCAS) – Qingdao, Shandong, China</footnote>

<footnote n="3">³ Department of Physical Geography and Ecosystem Science [Lund] – Lund, Suède</footnote>
# Reconstruction of intermediate and deep-water hydrology in the Pacific Ocean since the last glacial period

Xiaojie Tang¹, Giuseppe Siani¹, Christophe Colin¹

¹Géosciences Paris Saclay (GEOPS) – Université Paris-Saclay, CNRS – Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France

The Southern Ocean (SO) is a key area for global climate. It connects the deep water with the surface through upwelling, partly controlled by westerly winds. The deep SO, storing a large amount of carbon during the glacial period and releasing CO₂ during the deglacial period, is critical for the carbon cycle on orbital-millennial timescales.

Ocean circulation and stratification may differ between glacial and interglacial periods. In the Southeast Pacific (SEP), there are some reconstructions of intermediate and deep water, however, on the one hand, the reconstructions are mainly based on carbon isotopes. On the other hand, the available data is not enough to resolve spatial complexity and formation mechanisms of water masses.

Neodymium (Nd) isotopes in water mass is dominated by the crustal sources, which is spatial heterogeneous, enabling its wide application as water mass traces.

Here, we try to investigate the hydrological changes and the geometrical variations at intermediate to bottom depth (~1,500 to ~3,500 m) in the SEP since the late glacial (~50 kyr) by Nd isotopes (¹⁴⁴Nd/¹⁴³Nd), which is firstly used in the SEP.

Five cores along the south Chilean margin have been selected, core MD07-3100 (41°36′S; 74°57′W; 1609 m depth), core MD07-3098 (40°55′S; 75°01′W; 3030 m depth), core MD07-3119 (46°05′S; 76°06′W; 2523 m depth), core MD07-3081 (49°25′S; 76°58′W; 1620 m depth) and core MD07-3088 (46°06′S; 75°41′W; 1536 m depth). They are located within the boundary of PDW or the transition between PDW and CDW. Additionally, to better constrain the evolution of deep and intermediate water, three cores located in the South West Pacific and Indo-Pacific region were additionally added. For the core in the SWP, it's also located in the transition between Pacific deep water and circumpolar deep water. For the cores in the Indo-Pacific region, one core is located in the upper circumpolar deep water; the other core is located in the outflow of Indonesian through flow. Up to now, 85 samples in 4 cores have been processed.
# The Origin of Ferruginous Concretions on the Sunda Shelf and Its Environment Implications

Kaikai Wu<fnref n="1" />, Xuefa Shi<fnref n="1" />, Shengfa Liu<fnref n="1" />, Franck Bassinot<fnref n="2" />, Christophe Colin<fnref n="3" />, Hui Zhang<fnref n="1" />, Che Abd. Rahim Mohamed<fnref n="4" />

Marine concretions hold important resource, climatic, and environmental significance. A large number of studies have focused on continental and deep-sea Fe/Mn concretions. However, the genesis of ferruginous concretions on the continental shelf remains unclear. In this study, we present the radiocarbon ages (AMS ¹⁴C), elemental compositions, and mineral compositions of ferruginous concretions obtained from a gravity core on the central Sunda Shelf. The results indicate that the formation age of ferruginous concretions ranges from 10.5 to 7.5 cal ka BP from the bottom to the top. The predominant mineral identified in these concretions is goethite, which cements clay and other minerals in colloidal form. Notably, Fe, Si, and Al display distinct ring-band rhythmic variations. Our study suggests that ferruginous concretions are formed in subaerial and hydrogenetic environments, with a weathered stiff clay layer serving as the main stratum for concretion development. During periods of low sea levels (Before 10.5 cal ka BP), paleochannels likely provide fluctuating wet and dry conditions for the formation of light and dark concentric layers within ferruginous concretions. Between 10.5 and 7.5 cal ka BP, the Sunda Shelf experienced gradual inundation, leading to the partial transport of concretions from nearby channels. Subsequently, after 7.5 cal ka BP, the paleochannels became completely submerged, halting the formation of ferruginous concretions on the Sunda Shelf. Meanwhile, ferruginous concretions on the seafloor became mixed with sediments and marine organisms due to hydrodynamic sorting. The presence of ferruginous concretions can serve as indicators of paleochannel locations, aiding in the reconstruction of paleo-channel systems during periods of low sea level on the shallow continental shelf.

<footnote n="1">¹ Key Laboratory of Marine Geology and Metallogeny, First Institute of Oceanography — Ministry of Natural Resources Qingdao, China</footnote>

<footnote n="2">² Laboratoire des Sciences du Climat et de l'Environnement (LSCE) — CEA, CNRS, Université de Versailles Saint-Quentin-en-Yvelines — Université Paris-Saclay, 91191 Gif-sur-Yvette Cedex, France</footnote>

<footnote n="3">³ Géosciences Paris Saclay (GEOPS) — Université Paris-Saclay, CNRS — Université Paris Saclay, bâts. 504 & 510, 91405 Orsay Cedex, France</footnote>

<footnote n="4">⁴ Faculty of Science and Technology —National University of Malaysia —Selangor, Malaisie</footnote>