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As one of the most widespread components in marine sediments, calcium carbonate (CaCO3) plays a crucial role in the global carbon cycle and climate changes. To efficiently semi‐quantify the CaCO3 concentration, a more effective, non‐destructive, economic, and accurate technique is required. Diffuse reflectance spectroscopy (DRS) has been widely used for the detection and quantification of minerals but has been less studied for CaCO3. This study synthesized a series of samples with well‐determined content of CaCO3 to analyze the visible and near‐infrared DRS characteristics. Results show that the intensity of the second derivative of Kubelka‐Munk (K‐M) remission functions of the DRS spectra at 2,340 nm (I2340) is linearly correlated with the CaCO3 content. Then, the new proxy I2340 was applied for marine sediments from the Pacific Ocean with known CaCO3 concentrations determined previously by chemical methods, and the robustness of the I2340 as the proxy for CaCO3 concentration was attested. Plain Language Summary: Calcium carbonate (CaCO3) is one of the most common components in marine sediments, and its content variations (precipitation and dissolution) play an important role in global environmental change and paleoclimate evolution. However, previous quantitative methods such as chemistry and X‐ray diffraction cannot simultaneously achieve efficiency, accuracy, and environmental protection. To solve the above problems, this study established a new proxy using visible and near‐infrared diffuse reflectance spectroscopy (DRS) technology that can detect calcium carbonate content in a fast, non‐destructive, economic, and eco‐friendly way. The verification results from synthetic standard samples and natural marine sediment samples indicate that the new DRS proxy can well reflect the variations in calcium carbonate content, which is more targeted for calcium carbonate with less interference compared with the traditional brightness proxy. Overall, this study provides a promising method for precisely quantifying calcium carbonate in marine sediments. Key Points: CaCO3 exhibits a significant absorption in the near‐infrared band between 2,300 and 2,400 nmA new spectral proxy I2340 is proposed to quantify marine sedimentary CaCO3 contentI2340 is more accurate in quantifying marine sedimentary CaCO3 than traditional brightness proxy [ABSTRACT FROM AUTHOR]

The anhysteretic remanent magnetization (ARM) is an important magnetic parameter for quantifying the concentration of "fine", mostly sub‐micron ferrimagnetic particles in rocks and sediments. A sound understanding of ARM carriers is needed to interpret magnetic and environmental information in sedimentary archives. This is often not achieved for marine sediments, which routinely contain several magnetic mineral components. Here, we analyze marine sediment records over different timescales for four sediment cores from the Eastern Pacific Ocean (Hole 1218A), the Antarctic margin (core P4‐1), the Arctic Ocean (core ARC5‐ICE4), and the South China Sea (core L07), using coercivity spectra analyses of ARM, first‐order reversal curve principal component analysis (FORC‐PCA), transmission electron microscopy (TEM), and unmixing of isothermal remanent magnetization (IRM) curves. By combining rock magnetic and TEM results, we find that the ARM is mainly carried by non‐interacting single domain (SD) biogenic magnetite (magnetofossils) in the studied sediments over all timescales. Low‐coercivity magnetic particles (detrital and fine‐grained extracellular magnetite) also contribute to the bulk ARM. Variable magnetofossil chain structures are detected and have a significant influence on ARM data interpretation. Quantitative FORC‐PCA endmember analyses based on quantile contours and coercivity distributions provide valuable information on the nature of endmembers and improve ARM data interpretation. We provide an integrated strategy to help reduce interpretational ambiguities related to ARM in future studies. Plain Language Summary: The anhysteretic remanent magnetization (ARM) is an important parameter that is used to assess information about Earth's magnetic field variations and environmental changes. To interpret ARM signals robustly, we must understand the sedimentary magnetic minerals that control ARM changes. However, the origin of ARM changes in marine sediments is not well understood. Here, we analyze marine sediment records from the Eastern Pacific Ocean, the Antarctic margin, the Arctic Ocean, and the South China Sea to identify the carriers of ARM in the respective sediments. Our results indicate that all of the sediments contain abundant remains of magnetotactic bacteria (known as magnetofossils). Magnetofossil abundance variations dominate ARM on both 10,000 to million‐year timescales. Part of the bulk ARM is also contributed by other low‐coercivity magnetite particles. Furthermore, ARM is influenced significantly by magnetofossil chain structure variations. Our results have important implications for future studies involving use of ARM in magnetofossil‐rich sediments. Key Points: Integrated component analyses are presented for four sediment cores to identify magnetic particle sourcesMagnetofossils are the predominant anhysteretic remanent magnetization (ARM) carrier with detrital and extracellular magnetite also contributing to the ARMQuantitative analyses provide valuable insights into the nature of components to improve ARM data interpretation of marine sediments [ABSTRACT FROM AUTHOR]

The relationships among the abundance of magnetofossils, the ensuing magnetic properties, and the controlling paleoenvironmental factors in marine sediments remain broadly unexplored. Here, we identify magnetofossils in core XB1 from the northwestern South China Sea (SCS) since end of the Last Glacial. Using rock magnetic and electron microscopic data, we propose a model that links the anisotropy of magnetic susceptibility fabric and the abundance of magnetofossils. The magnetofossil concentration in sediments increases significantly during the 14.7–4.7 ka period, which in turns leads to inverse magnetic fabrics and near‐horizontal of the minimum magnetic susceptibility axes. Further, we show that the abundance of magnetofossils is linked to paleoenvironmental changes in the northwestern SCS. The production and preservation of magnetofossils during the 14.7–4.7 ka period are promoted by an intensified East Asian summer monsoon and sluggish deep‐water ventilation, while the paucity of magnetofossils after 4.7 ka is attributed to high oxygen content. Plain Language Summary: The anisotropy of magnetic susceptibility (AMS) is a useful magnetic property of rocks for a wide range of scientific questions. Typically, the sedimentary AMS fabric carried by coarser‐grained magnetic particles (i.e., multi‐domain, MD) exhibits an oblate shape with a horizontal maximum magnetic susceptibility axis. However, single‐domain (SD) magnetic particles (e.g., nano‐sized magnetofossil magnetites) complicate the AMS fabric, through their intrinsic inverse magnetic fabric with the minimum magnetic susceptibility axis aligned along the horizontal plane. For natural samples, quantification of the contributions of SD and MD particles to AMS is essential for accurate interpretation of the AMS. This paper puts forward a novel approach that links the AMS and the abundance of magnetofossils. By applying this new method to sediments from the South China Sea, we found that the inverse magnetic fabrics in the studied sediments are associated with high magnetofossils concentration, which can be affected by various factors including the East Asian monsoon, nutrient supply or deep‐water ventilation. Key Points: Values of the minimum susceptibility axis inclination are associated with the abundance of magnetofossilsThe magnetofossils can be quantified by the proportion of inverse component in sedimentsThe strong deep‐water ventilation hinders the production and preservation of magnetofossils in the South China Sea [ABSTRACT FROM AUTHOR]

Magnetotactic bacteria (MTB) require iron and organic carbon for biomineralization, which can be supplied by dust inputs that stimulate primary productivity in iron‐limited oceans. However, MTB activity and its relationship to Asian dust inputs and primary productivity are not well known in the North Pacific Ocean. Here, we present an eight‐million‐year record of magnetofossil abundance, Asian dust flux, and primary productivity from North Pacific sediments (Ocean Drilling Program Hole 885A). Our results demonstrate that Asian dust delivered iron to stimulate primary productivity and then boost MTB biomineralization in the late Miocene. Since the late Pliocene, dust inputs and primary productivity appear to have been decoupled, with MTB activity controlled mainly by primary productivity rather than dust inputs. Equant magnetofossil abundances co‐vary with eolian flux, and may provide here a proxy for dust inputs. This work provides perspectives on relationship between MTB activity, dust inputs, and primary productivity on million‐year timescales. Plain Language Summary: Magnetotactic bacteria (MTB) biomineralize chains of characteristic magnetically single domain magnetite (or greigite) crystals in their cells and can thrive in deep‐sea ecosystems and absorb iron for biomineralization. Magnetic particles that are preserved in sediments after MTB death are known as magnetofossils. Eolian dust supplied from continents is a major iron source in pelagic environments. Hence, magnetofossils can be important indicators of the impacts of dust‐supplied iron on deep‐sea ecosystems. A better understanding of the relationship between MTB activity, dust inputs, and primary productivity is needed for reliable magnetofossil interpretation. However, this relationship is not well determined in the North Pacific Ocean. By integrating rock magnetic, transmission electron microscope, and diffuse reflectance spectroscopy observations, we present an eight‐million‐year record of magnetofossil variations, Asian eolian inputs, and primary productivity from the North Pacific Ocean. Our results demonstrate that Asian dust inputs appear to have promoted MTB activity by stimulating surface primary productivity in the late Miocene. Since the late Pliocene, MTB activity was controlled mainly by primary productivity, with decoupling of the relationship between dust inputs and primary productivity. Moreover, equant magnetofossil abundances vary with eolian flux and provide a proxy for dust inputs in these marine sediments. Key Points: Asian dust inputs promoted magnetotactic bacterial activity by providing iron to stimulate primary productivity in late MioceneMagnetofossil production was controlled mainly by primary productivity changes rather than dust inputs since late PlioceneEquant magnetofossil abundances co‐vary with dust inputs and may be regarded here as a dust indicator [ABSTRACT FROM AUTHOR]

Gyro‐remanent magnetization (GRM) is a frequently occurring yet unwanted remanence contamination for certain samples during alternating field (AF) demagnetization of the natural remanent magnetization. The origin and detailed properties of GRM have not yet been fully understood. In this study, systematic rock magnetic analyses were conducted on marine greigite‐bearing samples of Hole U1433A drilled by the IODP Expedition 349 from the South China Sea. Results show that GRM is mostly acquired above ~55 mT AF demagnetization and can be effectively removed by heating to ~400°C during thermal demagnetization but a secondary tail could remain until ~585°C. In addition, no apparent GRM was observed during the AF demagnetization for the 400°C thermally treated samples. These results strongly suggest that GRM is dominantly carried by single domain (SD) greigite but with minor contributions from SD magnetite. Thus, thermal treatment alone or the hybrid demagnetization (i.e., thermal demagnetization at ~400°C first then systematical AF demagnetization) can efficiently avoid the GRM acquisition and be beneficial for relative paleointensity estimation for greigite‐bearing samples. Besides, GRM carried by greigite has a low thermal stability. Our results also show AF demagnetization spectra of anhysteretic remanent magnetization (ARM) could be strongly distorted by GRM effects due to both have a preference of SD particles. Thus, the median destructive field of ARM is improper to be used as a coercivity proxy for greigite‐bearing samples. Instead, the biplot analysis of AF demagnetization of natural remanent magnetization and ARM can be used to evaluate the relative content of greigite. Key Points: Both greigite and the coexistent magnetite can acquire a GRM during the AF demagnetizationMagnetite lost the ability to acquire a GRM when greigite is disintegrating after first being heated to 400°CGRM can be used as a proxy for semiquantification of greigite concentration [ABSTRACT FROM AUTHOR]

There are two distinctly different views on Quaternary sedimentation rates in the central Arctic Ocean, namely, that they were on cm/kyr or on mm/kyr scales, largely as a result of divergent interpretations of magnetic reversal stratigraphy. This study provides new evidence to help resolve this controversy in the form of an almost 1‐m‐thick negative polarity interval located at ~1.15 m below the seafloor in a 4.15‐m‐long sediment core from the Lomonosov Ridge, central Arctic Ocean. This thick polarity anomaly was first revealed through alternating field demagnetization of natural remanent magnetization on u‐channel samples. It was subsequently confirmed to be an authentic signal of a syndepositional geomagnetic field by integrating detailed rock magnetic studies, transmission electron microscopy with energy‐dispersive X‐ray spectroscopy analyses of magnetic extracts, and both alternating field and thermal demagnetization of discrete paleomagnetic samples. Here we conclude that it is more reasonable to interpret the reversed interval as a record of the Matuyama Chron rather than a Brunhes‐aged excursion, which yields mean sedimentation rates of ~1.5 mm/kyr and ~0.5 mm/kyr in the Brunhes and Matuyama Chrons, respectively. These estimates are remarkably lower than assumed in most recent studies, but in good agreement with both previous magnetostratigraphic and recent geochemical studies relative to the decay of Be and U‐series isotopes, which might indicate "sediment‐starved" environments at least in some parts of this region. Overall, our results not only imply that chronostratigraphy in the central Arctic Ocean remains open to debate but also warrant further investigations in this microfossil‐barren region. Key Points: The effects of self‐reversed CRM of diagenetic titanomaghemite are probably only regional in the central Arctic sedimentsThe "reversal" interpretation supports "sediment starved" environments at least in some regions of the central Arctic OceanThe chronostratigraphy and usability of paleomagnetism in the central Arctic Ocean remains open to debate and further studies are warranted [ABSTRACT FROM AUTHOR]

We investigated sea‐surface magnetic anomaly data for the past 5 Myr in the slow‐to‐intermediate spreading South Atlantic Ridge. After error analysis and signal processes, the sea surface magnetic anomaly profiles were downward continued to 1.5 km below sea level, and then the distance from spreading axis was converted to time by matching the observed anomalies with forward models of known polarity reversals. The stacked anomaly profiles from South Atlantic Ridge exhibit short‐wavelength wiggles superimposed on the first‐order variations of magnetic anomalies. The stacked anomaly profiles were first compared with commonly used geomagnetic palaeointensity data synthesized from sedimentary relative intensity records in the past and were further correlated with other available magnetic profiles derived from Central Indian Ridge, East Pacific Rise, and Pacific‐Antarctic Ridge. We found that short wavelength variations in the South Atlantic Ridge can be ascribed to geomagnetic intensity variations from excursions or short polarity events. Spectral analysis of marine magnetic anomalies records reveals that there is no obvious dominant periodicity for the geomagnetic field behavior during the past 5 Myr. Key Points: A long‐term paleointensity record was constructed from sea surface magnetic anomaly data at a slow‐to‐intermediate spreading SAR for the first timeThe newly stacked paleointensity curves agree well with other coeval relative paleointensity dataWe proposed a time scale based on the Earth's paleointensity from marine composite profile for the past 5 Myr [ABSTRACT FROM AUTHOR]

Abstract: Eolian dust is a major terrigenous component in North Pacific Ocean pelagic sediments and is an important recorder of Asian terrestrial environmental evolution and Northern Hemisphere atmospheric circulation. In order to extract and quantify wind‐borne mineral signals and develop a reliable eolian dust proxy for pelagic sediments, we investigated sediments from Ocean Drilling Program Hole 885A, North Pacific Ocean, by integrating results from environmental magnetism, transmission electron microscopy, and diffuse reflectance spectroscopy (DRS). Our results indicate that ferrimagnetic and antiferromagnetic minerals coexist in the sediments. The former includes biogenic magnetite, nano‐sized single domain (titano) magnetite inclusions embedded in silicate hosts, submicron vortex state magnetite, and ultrafine superparamagnetic maghemite. The latter includes hematite and goethite. Our results indicate that magnetic properties can be used to determine an overall trend of increased eolian dust inputs, with additional significant influence from biogenic and volcanogenic components. In contrast, a newly proposed parameter RelHm+Gt (combined hematite and goethite concentration) obtained from DRS measurements extracts precisely the eolian signal and precludes other components. Our DRS results indicate that hematite/goethite and siliceous fractions deposited at Hole 885A were jointly delivered to the North Pacific Ocean from Asian dust sources. Therefore, we suggest that RelHm+Gt is a reliable eolian dust proxy for pelagic sediments in this region. [ABSTRACT FROM AUTHOR]

Magnetic minerals in sediments of the continental shelf are sensitive to environmental changes. Therefore, to determine the exact phases of magnetic minerals is the prerequisite to investigate the paleoenvironmental significances. In this study, a comprehensive rock magnetic investigation, coupled with mineralogical studies, was carried out on the long core NHH01 (with a length of 125.64 m) from the central continental shelf of the South Yellow Sea. Single-domain greigite was identified as the dominant magnetic carrier in sediments between 44.90 and 51.80 m, and terrigenous magnetite is the major magnetic phase in the adjacent layers. The existence of greigite unambiguously indicates an anoxic environment. We tentatively interpreted that such an environment could be formed by the fast sea level changes, long-existing of the cold eddies during warm stages, or in a lake which might be related to locally tectonic uplifting. Therefore, the occurrence of the thick-layer greigite could supply a new perspective to paleoenvironmental or even tectonic studies. [ABSTRACT FROM AUTHOR]

SUMMARY We report high-resolution palaeomagnetic results across the lower part of S8 in the Luochuan area, northwest of China using parallel subsets samples. A palaeomagnetic anomaly with low palaeointensity and disordered magnetic direction was identified ∼21 ka prior to the Matuyama-Brunhes (MB) polarity reversal, and is consistent with the MB precursor observed from marine sediments and lava flows. This is the first convincing report of the MB precursor in terrestrial material (Chinese loess), which attests the global feature of the MB precursor. Constrained by the stratigraphic position of both the MB polarity reversal and the MB precursor, the loess unit L8 and palaeosol unit S8 of the Chinese loess sequence are undoubtedly tied to the marine isotope stages 18 and 19, respectively. This correlation is critical for an accurate understanding of the correspondences of palaeomagnetic records between the Chinese loess and marine sediments. [ABSTRACT FROM AUTHOR]

Aeolian dust, a primary terrigenous component of ocean sediments, has been widely used to reconstruct the paleoclimatic evolution because its transported distance, grain size and concentration are sensitive to climate changes. To further characterize the aeolian dust, the deposits at site Ocean Drilling Program (ODP) 882A in northwestern Pacific Ocean are divided into four grain-size fractions (<8, 8-16, 16-64, >64 μm) using the gravitative differentiation method. Detailed rock magnetism results show that magnetite and hematite are dominant magnetic minerals for the dust components. In addition, the aeolian dust (<8 μm) represented by the concentration of magnetic minerals increases sharply at 2.73 Ma, which corresponds to the onset of major glaciation in the Northern Hemisphere. In contrast, the ice-rafted detritus (IRD) (>64 μm) contributes little to the magnetic enhancement of the sediments at 2.73 Ma. These new results greatly improve our understanding of paleoenvironmental evolution during late Pliocene-early Pleistocene in this area. [ABSTRACT FROM AUTHOR]

Abstract: Chinese loess during the Quaternary is important for both paleomagnetic and paleoclimatic studies. Although magnetostratigraphy has been used for the initial loess timescale, paleomagnetic records in loess still remain in debates, such as magnetic polarity anomalies in L9, which is a thick and coarse sandy horizon as a typical marker bed. To better understand the magnetization mechanism of these anomalies, a new age model for L9 was first constructed based on a new correlation between loess and marine δ18O, which proposed that L9 should be correlated to marine oxygen stage (MIS) 20–24, not 22–24 as previous suggested. Using combined grain size and environmental magnetism parameters, an extended climatic optimum corresponding to MIS 21 can be identified in middle of L9. A remagnetization mechanism of viscous remanent magnetization overprinting on characteristic remanent magnetization (ChRM) carriers (mainly large pseudo single domain and multidomain magnetite particles), associated with the chaotic detrital remanent magnetization caused by low efficiency alignment of ChRM carriers along with the ambient field during deposition, is proposed to interpret the magnetic anomalies. This study provides new insights into the magnetization mechanism of the Chinese loess, and the new age model for L9 helps to accurately refine the finer scale correlation of paleoclimatic records between the Chinese loess and marine sediments. [Copyright &y& Elsevier]

Abstract: It is commonly accepted that the climate records in Chinese loess can be correlated well with that in marine sediments. However, discrepancies for the stratigraphic position of paleomagnetic polarity reversal boundary seriously restrict an accurate teleconnection between these two archives. For example, the exact stratigraphic position of the whole Matuyama–Brunhes (MB) transitional polarity zone remains uncertain in Chinese loess. In this study, an accurate transitional zone of the MB reversal from the Mangshan profile, southeast margin of the Chinese Loess Plateau (CLP), was statistically determined using multiple subsets of parallel samples. By integrating results from the central CLP, the whole MB transitional zone is consistently located across the pedostratigraphic and climatostratigraphic transitional zones between S8 and L8 over a wide region of the CLP. This conclusion further supports that the paleosol unit S8 should be teleconnected to the marine oxygen isotope stage 19 rather than 21, and thus unambiguously supplies an accurate age control in constructing a new loess time scale. [ABSTRACT FROM AUTHOR]

Abstract: Bioturbation disturbs detrital magnetic particles after deposition, so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post-depositional remanent magnetization (PDRM) whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle. The relationship between the Matuyama–Brunhes boundary (MBB) and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked-in. However, such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location. It is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis. When the global data set is subjected to such a rigorous analysis, few reliable data remain for the MBB. Using two complementary approaches, we estimate that the MBB is, on average, shifted ≤20 cm below its true position in marine sediments. This offset is the sum of the thickness of the bioturbated surface mixed layer, which is possibly dominant, and the PDRM lock-in depth. There is also controversy concerning observed differences in the position of the MBB relative to paleoclimatic proxies in marine sediments and Chinese loess deposits. For the Chinese loess, quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the MBB with respect to paleoclimatic boundaries. We conclude that the MBB occurs late in marine oxygen isotope stage 19, and in the upper part of Chinese paleosol unit S8, rather than at the mid or lower part of loess unit L8. Our results require adjustment of the generally accepted positions for the MBB, and resolve a longstanding chronological conundrum for marine and Chinese loess sequences. [Copyright &y& Elsevier]

Contourites are globally-distributed features along continental margins where deep-water masses come across the seafloor; however, the genesis and metal-sources of marine mineral deposits associated with contourite deposits remain largely unknown. Abundant Fe-rich nodules were recovered along the continental margin of the South China Sea, where extensive cold-seep systems were also discovered. We measured synthetically geochemical elements, δ13C, Sr and Nd isotopic data and organic proxies in Fe-oxy-hydroxide deposits from the upper and lower slope. Fe-rich nodules recovered from the upper slope represent the effect of an integrated diagenetic-hydrogenetic formation environment, in which fluid venting from deep-seated hydrocarbon reservoirs and deep-water erosion and chemistry could play an important role. In contrast, another kind of ferromanganese nodule forming along the lower slope and predominantly hydrogenetic is connected to a distinct deep-water flow from the North Pacific. These results give us a new insight into the mineralization processes associated with Fe oxidation-reduction processes on continental margins affected by contourite systems with abundant hydrocarbon seep, bottom water, and terrigenous inputs. • Fe-Mn nodules and Fe-rich nodules co-exist with abundant hydrocarbon-derived chimneys and crusts at the Dongsha Islands. • Fe-Mn nodules from lower slope are influenced by alternating episodes of burial and exhumation due to Pacific Deep Water. • Biogeochemical processes are involved in Fe reduction in the South China Sea continental shelves. [ABSTRACT FROM AUTHOR]