Increased terrigenous input from North America to the northern Mendeleev Ridge (western Arctic Ocean) since the mid-Brunhes Event

Mid-Brunhes Event (MBE) occurred at approximately 420 ka between Marine Isotope Stage 11 and 12, and is considered the most pronounced climatic shift during the last ~ 800 kyrs. On the other hand, it is unclear if the MBE was global, despite being observed in the high-latitude Northern Hemispheric cryosphere in terms of climate systems. A 5.35-m long gravity core ARC5-MA01 was obtained from the northern Mendeleev Ridge in the western Arctic Ocean to track the paleoenvironmental changes in terms of the terrigenous sedimentation in response to the glacial-interglacial climate changes across the MBE. Geochemical proxies (biogenic opal, total organic carbon, C/N ratio, carbon isotope of organic matter, and calcium carbonate) of MA01 suggest that the terrigenous input was generally higher during the interglacial periods. Based on a mineralogical examination, most of the terrigenous input was attributed to the abundance of dolomite and the increased kaolinite content from North America. In particular, most paleoceanographic proxies showed that the terrigenous input from North America was enhanced distinctly during the post-MBE interglacial periods. These results suggest that the MBE in the western Arctic Ocean was a global climatic shift closely linked to cryospheric development in North America during the middle Pleistocene.

www.nature.com/scientificreports/ across the MBE in the western Arctic Ocean, related to a decrease in organic matter export and/or weakened bottom water ventilation along with the development of perennial sea ice. In general, the climatic shift across the interglacials-terminations since the MBE was intensified clearly, but the climate changes during the glacial periods across the MBE are poorly resolved. Such a lack of information is typical for the Arctic Ocean, where calcareous microfossil preservation is poor especially during glacial periods 10 . Given that the predominent growth of Northern Hemisphere glaciation played an important role in changing the global climate across the Pleistocene 11 , continuous climate records from the Arctic Ocean are a critical subject for unraveling the Arctic glacial history. In particular, the changes in the G-IG amplitudes during the Pleistocene involve various internal forces, including the hypothesis of regolith removal caused by the continental glaciation 11,12 . In this study, we examined geochemical and mineralogical proxies of core ARC5-MA01 (hereafter MA01) in terms of terrigenous sedimentation to reveal the paleoenvironmental changes by orbital-scale glacial activities during the last ~ 840 krys in the northern Mendeleev Ridge of the western Arctic Ocean. Our results reveal an increase of North American terrigenous input to the western Arctic Ocean since the MBE, supporting an global climatic amplification since the MBE.
Based on the carbonate quantification, most TIC consists of dolomite during the last ~ 840 kyrs (Figs. 2 and 4). The calcite accounts partly for TIC during warm periods since MIS 7, which corresponding to the peaks of planktonic foraminiferal abundance (Fig. 4). In addition, CaCO 3 peaks associated with dolomite peaks during MIS 16,13,7,5, and 3 coincide with the intervals of high kaolinite content and coarse fraction (Fig. 4).

Discussions
Glacial-interglacial changes of geochemical and mineralogical proxies in the northern Mendeleev Ridge during the last 840 kyrs. Geochemical proxies of core MA01 are characterized by G-IG cycles and, particularly, by long-term changes during the last ~ 840 kyrs (Fig. 2). The biogenic opal content, representing the primary productivity of the surface seawater, is fairly low (3.1% in average), which is attributed to the year-round sea ice cover in the central western Arctic Ocean 13,14 . Such low biogenic opal content is similar to those in cores from the central-to-eastern Arctic Ocean (~ 2-4% in average) 13 and from the Chukchi Rise (3.7-4.4% in average) 15 , which is located northwest and southeast to the Mendeleev Ridge, respectively. In the Northwind Ridge, Polyak et al. 14 , based on foraminiferal assemblages, reported that perennial sea ice condition was established at ~ 0.8 Ma. This is consistent with the overall low primary productivity in this study, indicating that the northern Mendeleev Ridge was covered dominantly with perennial sea ice during the last ~ 840 ka. Despite low, biogenic opal content in this study is slightly different between glacial and interglacial periods, particularly higher (up to ~ 7.4%) during MIS 5, 7, and 13 ( Fig. 2). Such difference was also reported from the central Arctic Ocean 13 and the Chukchi Rise 15 . Schubert and Stein 13 suggested that the low biogenic opal content during glacial periods was attributed to both reduced marine productivity and increased input of terrigenous sediments. The lower biogenic opal content in the northern Mendeleev Ridge during the glacial periods can be also attributed to the similar reasons of Schubert and Stein 13 . Thus, based on the occurrence of planktonic foraminiferal tests in MA01 which is in line with the G-IG variability of biogenic opal content (Figs. 2 and 4), surface water conditions in the northern Mendeleev Ridge changed between the glacial and interglacial periods during the last ~ 840 kyrs.
TOC content and C/N ratio also represent G-IG changes related to surface water conditions in the northern Mendeleev Ridge (Fig. 2 and Table 2). Similar to the biogenic opal content, TOC content exhibits G-IG variability that is characterized by high content during interglacial periods and low content during glacial periods. The low C/N ratios are mostly due to low TOC content and high proportion of the inorganic nitrogen dominating the  16 . The strong possitive correlation (r 2 = 0.88 for interglacial periods and r 2 = 0.97 for glacial periods) between TOC contents and C/N ratios indicates that higher TOC contents were attributed to more contribution of terrestrial organic matters rather than an enhanced surface water productivity in the northern Mendeleev Ridge (Figs. 2 and 3). Such increased contribution of terrestrial organic matter in the northern Mendeleev Ridge is supported by generally low δ 13 C org values (Fig. 2). Previous studies reported that the input of terrestrial organic matter was higher during glacial periods in the western Arctic Ocean 17,18 . In the southern Mendeleev Ridge, Yamamoto and Polyak 17 suggested that the increased input of terrestrial organic matter during the cold periods was associated with more efficient transport of glaciogenic fine-grained particles from the continent to the western Arctic Ocean. In the Chukchi Borderland, the increase of TOC content was mostly associated with low coarse-grained (> 150 μm) fraction during the cold periods, whereas the marine production of organic carbon more likely increased during the warm periods 15,18 . In contrast, our record from the northern Mendeleev Ridge shows higher TOC content during the interglacial periods rather than the glacial periods (Figs. 2 and 4). Rella and Uchida 18 reported that TOC content was negatively correlated with CaCO 3 and coarse fraction contents in the Chukchi Borderland. For example, high TOC content was accompanied with low CaCO 3 and coarse fraction contents, resulted from the transport of terrestrial organic matter from CaCO 3 -poor areas such www.nature.com/scientificreports/ as Siberian margins and/or the less transport of CaCO 3 to the Chukchi Borderland. In contrast, our geochemical results show that CaCO 3 content was positively correlated with both TOC content (r 2 = 0.55-59) and C/N ratio (r 2 = 0.60-70) (Fig. 3), indicating that both carbonates and organic carbon in the northern Mendeleev Ridge are mainly terrigenous in origin, similar to the study in the eastern-central Arctic Ocean 13,19 . Such terrigenous contribution is evidenced by the peaks of coarse fraction and dolomite contents (Figs. 2 and 4). It indicates that inorganic carbon components consisting mostly of dolomite mainly originated from the Canadian Arctic Archipelago and Mackenzie watershed 9,15,20,21 . Thus, the terrigenous input in MA01 is sourced dominantly from the northern North America. Our finding suggests that the northern Mendeleev Ridge in the western Arctic Ocean is largely affected by sediment deposition from North America. Such increased terrigenous input requires an enhanced Beaufort Gyre in the western Arctic Ocean, leading to more transportation of sediments from North  The strong G-IG variabilities are also observed in the variation of major clay mineral compositions of MA01 (Figs. 4 and S1). The overall predominance of illite is well accorded with the result of the previous studies in the western Arctic Ocean 15,21 . Nonetheless, the other minor clay minerals have been used for the provenance studies 15,21,23 . Smectite content in the western Arctic Ocean is very low due to its long distance from the main source areas in the eastern Arctic margins 15,21,24 , although the content is relatively high in the central-western Arctic Ocean 23,25 . Similar to this study in the northern Mendeleev Ridge, very low smectite content was reported from the Chukchi Rise (7-9% in average) 15 and Canada Basin (3% in average) 21 in the western Arctic Ocean.   www.nature.com/scientificreports/ Despite low content, the smectite content of MA01 seems different between the glacial and interglacial periods since the MBE (Fig. 4).
In the Chukchi-Alaskan Slope, chlorite was used as a proxy for the Pacific inflow through the Bering Strait 26 . However, chlorite is abundant at the East Siberian margin 16 . Due to the wide distribution of chlorite along the continental margins of the western Arctic Ocean, its G-IG cycles were less detectable in the Chukchi Rise 15 and Canada Basin 21 and the northern Mendeleev Ridge of this study area ( Fig. 4 and Table 2). In comparison, kaolinite in the western Arctic Ocean reflects a specific source area, i.e., northern Canada and Alaska 27 , and its content was fairly variable among the clay mineral assemblage in the Chukchi Rise and Canada Basin 15,21 . Similarly, the kaolinite content of MA01 shows strong G-IG variability (Figs. 4 and 5; Table 2). Recently, Xiao et al. 23 argued that the kaolinite was transported from the Franz Joseph Land in the Barents Sea to the Makarov Basin at several glacial/deglacial events including MIS 6 and 4/3. Such increase of kaolinite content was observed at MIS 4/3 but not at MIS 6 in MA01 (Fig. 4), presumably due to the different sedimentation regimes between the northern Mendeleev Ridge and the Makarov Basin. It should be noted that distinct kaolinite peaks of MA01 coincided with the detrital carbonate peaks (e.g., calcite, dolomite, and XRF-Ca/Al) indicative of the enhanced terrigenous  www.nature.com/scientificreports/ input from North America during MIS 16,13,7,and 5 (Figs. 2 and 4). All these imply that the clay-sized particles were also delivered together with the coarse fraction from North America to the northern Mendeleev Ridge. Park et al. 15 suggested that high kaolinite content in the Chukchi Rise was attributed to more terrigenous input from North America. The terrigenous input was more active during the deglacial period rather than the peak glacial period when the terrestrial glaciation and sea ice were most intensive 15,28 , which strongly reduced sediment transportation to the western Arctic Ocean 29,30 . Accordingly, in the northern Mendeleev Ridge, terrigenous input increased during the deglacials and/or interglacial periods when more terrigenous sediments could be transported efficiently from North America via iceberg and/or sea ice under open water condition.
Increased terrigenous input to the northern Medeleeve Ridge since the MBE and its paleoenvironmental implications. The geochemical and mineralogical results of MA01 demonstrate an amplification of G-IG contrast across the MBE (Fig. 5 and Table 2). Particularly, TOC and CaCO 3 contents, C/N ratio, and kaolinite content associated with dolomite peaks were distinctly high during the post-MBE interglacial periods, suggesting enhanced terrigenous inputs from North America (Figs. 2, 4, 5 and Table 2). According to terrestrial and marine records as well as modeling results, continental ice sheets in North America have grown remarkably during the late Quaternary [31][32][33][34] . In the northern North Atlantic, glaciogenic marine sediments were observed since the Pliocene-Pleistocene transition, while the detrital carbonate sediments from North America started to appear clear since MIS 16 35 . Similarly, in the western Arctic Ocean, detrital carbonate occurred distinctly since MIS 16, indicating more intensive expansion of North American ice sheets 9,36 . Hence, the sediment input originated from North America into the western Arctic Ocean increased during the middle Pleistocene as a result of the development of the North American ice sheets. Such increased terrigenous input to the northern Mendeleev Ridge is evidenced by our multiproxy results that are characterized by more distinct G-IG fluctuation since the MBE (Fig. 5 and Table 2), while the first extensive development of North American ice sheets occurred at MIS 16 (Fig. 4).
Many records regarding the climatic shift during the post-MBE (since MIS 11) are distinguished by strong interglacial conditions represented by warmer seawater temperatures, higher atmospheric CO 2 concentrations, and less ice volume 1,4-6,37 (Figs. 2 and 4). Such climate shift was also accompanied with strong glacial conditions including colder atmospheric temperature and lower CO 2 concentration 4,5 (Figs. 2, 4). A possible explanation to the MBE is related to the gradual removal of continental regoliths in the Northern Hemisphere during the Quaternary 11,12 . Regolith removals, resulting from the repeated glaciations, facilitated silicate weathering of the freshly exposed bedrocks and, thus, accelerated the removal of atmospheric CO 2 concentration, which subsequently enhanced global cooling by positive feedback 11,12 . Clark et al. 11 demonstrated that the long-term climate change superimposed G-IG cycles during the Quaternary is closely linked to the increase in ice volume since the early Pleistocene. This successive climatic evolution eventually occurred globally as the change of G-IG cycles from ~ 41 to ~ 100 kyrs that completed during the middle Pleistocene (~ 700 ka) 11 . Based on the geochemical and mineralogical proxies of MA01, such climatic changes related to ice sheet development on North America occurred first in the northern Mendeleev Ridge at MIS 16 (~ 640 ka) and fluctuated distinctly since the MBE (Figs. 2, 4, 5). Hence, because the terrestrial-origin sediment deposition increased since the MBE as a result of the development of the North American ice sheets adjacent to the western Arctic Ocean, our results support the regolith removal hypothesis 11,12 . Then, the remaining question is why more terrigenous sediments were deposited during interglacial periods in the northern Mendeleev Ridge of the western Arctic Ocean.
The cooling in the northern Mendeleev Ridge across the MBE can be found not only by the increase of terrigenous input but also by the change of the surface water condition. Despite the cause of the changes in calcareous microfossil abundance in the central-to-western Arctic Ocean remains unclear 38,39 , distinct G-IG contrast of surface water conditions are inferred by biogenic opal content and planktonic foraminiferal abundance of MA01 since MBE (Figs. 2, 4, 5). The G-IG contrast was particularly amplified since MIS 11 with a decrease (2.7% in average) of biogenic opal content during the glacial periods ( Fig. 5; Table 2), which indicates post-MBE colder glacial conditions. Such productivity difference seems to be related to the change in seawater temperature and sea ice cover between the glacial and interglacial periods. Cronin et al. 40 suggested that perennial sea ice started to develop during the interglacial period across the MBE based on calcareous microfossil assemblages that represented the faunal transition in the western Arctic Ocean. Later, thick perennial sea ice conditions were settled during the glacial periods 40 , corroborating a gradual cooling regime across the MBE in the northern Mendeleev Ridge. Based on the changes of microfossil abundance in the central-western Arctic Ocean, Marzen et al. 39 reported that sea-ice related productivity increased during the interstadial periods since the MBE, with modulation by ~ 100 kyrs of G-IG cycles. It suggests that the climate variability was more sensitive in the central-western Arctic Ocean during the strong interstadials compared to the global climate variablity 39 , as an example of Arctic amplification at geological time-scale 7 . Due to this high sensitivity to climate change, not only 100 kyrs cycle of G-IG contrast but also shorter (< 20 kyrs) stadial/interstadial variations are recorded in the northern Mendeleev Ridge, leading to more terrigenous input during the interglacial periods. Such high sensitivity to climate changes from global cooling eventually occurred as distinct G-IG contrast in the northern Mendeleev Ridge since the MBE. Continuous regolith removal during the Quaternary has been accelerated since the middle Pleistocene 37 . Such condition continued to supply more terrestrial sediments to the northern Mendeleev Ridge in the western Arctic Ocean during warmer and longer post-MBE interglacial periods and multiple stadial/interstadial transitions.

Conclusions
In this study, diverse geochemical and mineralogical proxies of MA01 were analyzed to track paleoenvironmental changes in terms of terrigenous sediment deposition related to the glacial history during G-IG climatic changes for the last ~ 840 kyrs. The overall features of geochemical and mineralogical results in the northern Mendeleev Ridge accorded with the previous results reported in other regions of the western Arctic Ocean. But partial difference from the central-western Arctic Ocean seems attributable to the regional difference in depositional environment. Our results highlighted that the most significant difference between the glacial and interglacial periods was the terrigenous input from North America and sea surface conditions during the post-MBE. This finding is well aligned with the regolith removal hypothesis, which has been proposed as one of the primary causes of global cooling and completion of ~ 100 G-IG world during the Pleistocene. We suggest that the post-MBE G-IG contrast in the northern Mendeleev Ridge resulted from global cooling and internal positive feedback, both of which are closely related to the cryospheric development in North America. Although the strong G-IG contrast during the post-MBE seems observable in the northern Mendeleev Ridge, diverse regional effects should be considered using further paleoceanographic proxies for better understanding the paleoenvironmental history in the western Arctic Ocean.

Materials and methods
A 5.35 m-long gravity core MA01 was taken from the northern Mendeleev Ridge (178° 58′ E 82° 02′ N, 2295 m deep) by R/V Xue Long during scientific cruise CHINARE-V in 2012 (Fig. 1). Core sediments were subsampled at 2 cm intervals at Tongji University (China) and ground after freeze-drying at Pusan National University (Korea) to proceed the laboratory analyses. Xiao et al. 9 reported color reflectance, bulk elemental composition, coarse fraction (> 150 μm), foraminiferal abundance, AMS 14 C dating, and paleomagnetic inclination of MA01.
(Chrono-) stratigraphy. The uppermost intervals (0-23 cm) of core MA01 were constrained by AMS 14 C dates to MIS 1-3. For the older sediments, the stratigraphy was determined by lithostratigraphic and biostratigraphic regional correlations and tunning of Mn content to global climate variability 9 . The core stratigraphy was correlated to the earlier published records from a wide area of central-western Arctic Ocean, including the Mendeleev and Lomonosov Ridges and the Makarov and Canada Basins. The lowermost biostratigraphic marker constrains MIS 11 9 . Beyond MIS 11, the stratigraphy largely relies on the Mn-based tunning due to the lack of identifiable stratigraphic marker. Thus, the stratigraphy below MIS 11 interval seems tentative. Nonetheless, several lithostratigraphic features comfirm that the stratigraphy of MA01 is plausibly acceptable, such as the first detrital carbonate occurrence corresponding to MIS 16 35 and the distinct cyclic Mn variabilities, general lithologic feature of middle-to-late Pleistocene sediments in the Arctic Ocean 14,36 . In this study, we adopted the same age model of Xiao et al. 9 to interpret our geochemical and mineralogical data of MA01.
Geochemical measurements. Geochemical properties were analyzed on a total of 271 samples. Biogenic silica (Si BIO ) content was measured using the molybdate blue spectrophotometer following the wet-alkaline sequential extraction method modified from DeMaster 41 . The biogenic opal content was calculated by multiplying Si BIO by 2.4 42 . The analytical error of biogenic opal content is less than ± 1%. Total inorganic carbon (TIC) content was measured using UIC CO 2 Coulometer CM5014. The analytical error of TIC is ± 0.1% as relative standard deviation. CaCO 3 content was calculated by multiplying TIC contents by 8.333 (the ratio of C to CaCO 3 ). Total carbon (TC) and total nitrogen (TN) contents were measured using an elemental analyzer Flash 2000. The analytical errors are less than ± 0.1% and ± 0.01%, respectively. TOC content was calculated as the difference between TC and TIC and C/N ratio was calculated by TOC/TN. Carbon isotope of sediment organic matter (δ 13 C org ) was measured from 82 horizons using Europa Scientific 20-20 Elemental Analyser-Isotopes Ratio Mass Spectrometer (IRMS) at Iso-Analytical Ltd (UK). Expressed conventional delta notation is per mil deviation from the Vienna Pee Dee Belemnite (V-PDB) for carbon isotope. Precision for carbon isotope is ± 0.1‰.

Clay mineral analysis.
Major clay mineral compositions (smectite (17 Å), illite (10 Å), kaolinite, and chlorite (7.1 Å)) of fine-grained sediments were measured for 90 samples. Organic matters of bulk sediments were removed by 6% hydrogen peroxide solution. After wet-sieving through a 63 μm mesh, clay particles (< 2 μm) were extracted by settling technique based on Stoke's Law. The extracted clays were applied on glass slide into orientation and air-dried 43 . The dried slides were scanned using X-ray diffractometer (XRD; SIEMENS/BRUKER D5005) at Gyeongsang National University (Korea) and repeated after ethylene glycol-saturating at 60 °C during 24 h 44 . Relative contents of four major clay minerals were calculated semi-quantitatively following the scheme of Biscaye 45 . The proportion of kaolinite and chlorite was determined by their peak areas (3.58 Å and 3.54 Å), respectively 46 .
Quantitative analysis for carbonates. Contents of carbonate minerals (calcite and dolomite) were quantified for the selected 14 samples that were decided by the prominent peaks of coarse fraction and TIC content. Bulk sediment powders were analyzed using the same XRD. Following the assumption that the carbonate minerals of the Arctic Ocean sediments consist mostly of calcite and dolomite 34

Data availability
All data generated or analysed during this study are included in this published article and its supplementary file (MA01_dataset_202201_SR.xlsx).