Solar Cycles Forced Southern Westerly Wind Migrations During the Holocene

Despite small direct changes to radiative forcing, solar sunspot cycles are observed in climate records because of climate system amplification that primarily affects wind and precipitation belts. We present a proxy record resolving the dominant sub‐millennial periodicities across the entire Holocene in the Southern Westerly Winds (SWW), whose migrations are linked to ocean‐atmosphere heat and carbon exchange. We use X‐ray fluorescence core scanning to examine a rapidly accumulating sediment record (6 m/kyr) recovered from the Chilean margin, yielding unprecedented <2‐year resolution for the Holocene. We show that variations in terrigenous inputs to the site are linked to precipitation, which is controlled by SWW latitudinal migrations. Superimposed on a long‐term decreasing trend throughout the Holocene, we detect significant centennial cycles in the terrestrial input consistent with solar periodicities. We then propose a mechanism by which southward (northward) SWW movement in response to increasing (decreasing) total solar irradiance cools (warms) Antarctic temperatures.

. West of the Andes, the SWW are the primary precipitation source, providing ∼7,500-10,000 mm of rainfall per year (Garreaud et al., 2013). Along southern South America, the heavy precipitation results in runoff into the Pacific Ocean averaging 21 km 3 yr −1 (Lamy et al., 1998). At 30°S, north of the region under the influence of the SWW, annual rainfall is an order of magnitude lower. Better constraints on the range of natural variability in the SWW over the current interglacial period are needed for contextualizing modern climate conditions, for assessing the impact of solar-forced variability on climate, and for improving future climate projections.
SWW strength and position can also influence deep ocean heat and carbon storage as well as surface and subsurface heat and nutrient transport through their effect on the position of the oceanographic boundaries in the Southern Ocean and the strength of the Antarctic Circumpolar Current (ACC) (Sallée, 2018;Toggweiler et al., 2006). Pacific Deep Water (PDW) outcrops within the ACC and is pushed northward in the upper ocean by wind-driven Ekman transport, promoting further upwelling (Talley, 2013).
In this study, we take advantage of 65 m of Holocene sediment, from the Chilean Margin with subdecadal resolution to evaluate SWW variability and determine the impact of solar forcing on the SWW belt over the entire Holocene period. The sediment record reveals an evolution of dominant periodicities across the Holocene, some of which correspond to changes in TSI, indicating solar forcing of SWW migrations that we then link to Antarctic temperatures.

Materials and Methods
Expedition 379T, a non-IODP expedition aboard the D/V JOIDES Resolution, recovered ∼65 m long sediment core J1004 (44.0005°S, 75.1511°W, 1,124 m water depth). Located on a topographic high off the main channel of a small submarine canyon and minimally affected by bottom water currents, the site is an efficient trap for sediments delivered from land ( Figure S1 in Supporting Information S1).
An age-depth model was generated using nine radiocarbon dates on mixed planktic foraminiferal assemblages (Table S1 and Figure S2 in Supporting Information S1). Radiocarbon ages, converted to calendar ages using the Marine20 calibration curve within the Bchron R package with prescribed constant 541 ± 43-year reservoir age, yield a near constant sedimentation rate of 5.9 m/kyr throughout the Holocene (Haslett & Parnell, 2008;Heaton et al., 2022;Merino-Campos et al., 2018). The bottom of J1004 is composed of 5-10 cm diameter pebbles and transitions abruptly to silty clay at ∼11.4 ka ( Figure S3 in Supporting Information S1). Given the estimated bottom age, the transition from pebbles to silty clay likely indicates a local meltwater event contemporaneous with the end of the Younger Dryas in the Northern Hemisphere.
Seasonal migrations of the SWW result in high austral summer precipitation at this location along the central Chilean Margin and consequently fresh continental runoff (Figure 1 and Figure S4 in Supporting Information S1). Additionally, at 1,124 mbsl, J1004 allows for reconstruction of PDW, using benthic stable isotope measurements ( Figure S4 in Supporting Information S1).

XRF Analysis
Downcore variations in the bulk sediment geochemistry were analyzed via high resolution X-ray Fluorescence (XRF) using an ITRAX XRF core scanner (Supporting Information S1). Point measurements were made every 1 cm yielding ∼2 years resolution. The XRF data were converted to concentrations by comparison with discrete samples analyzed by inductively coupled plasma-optical emission spectrometry providing accuracy within ±10% for all major terrestrial elements, determined by comparison with certified reference materials AGV-2, BCR-1, MAG-1, and JG-2 ( Figure S5 in Supporting Information S1) (Bertrand et al., 2014;Murray et al., 2000).

Isotopes
Benthic foraminiferal species Uvigerina peregrina from the >125 μm size fraction were sonicated in MilliQ water to remove debris prior to analysis for δ 18 O and δ 13 C on a Micromass Optima mass spectrometer with a   of February and August precipitation from the Chilean Margin demonstrating seasonal Southern Westerly Winds and precipitation migration (Adler et al., 2018;Thyng et al., 2016). Site J1004 (44.0005°S, 75.1511°W), indicated by a red and white star, is plotted with data from the Global Precipitation Climatology Project Monthly Precipitation Climate Data Record provided by the NOAA PSL from their website https://psl.noaa.gov.

Statistics
Statistical assessment of potential quasiperiodic variability in the data series was conducted using multitaper method power spectral analysis (MTM, Thomson, 1982), and mutitaper Evolutive Power Spectral Analysis (EPSA), as implemented in the software Astrochron (Meyers, 2014, functions "mtmML96" and "eha"). We utilize the Bonferroni multiple-testing correction to evaluate statistical significance of spectral peaks (Vaughan et al., 2011). Prior to analysis, all data series were placed on an even sampling grid with piecewise linear interpolation, using the median sampling interval of the given series (function "linterp"). We identify and eliminate extreme values in the J1004 wt.% Al series if they were 1.5 times lower than the first quartile (25%), or 1.5 times greater than the third quartile (75%), using the "trim" function in Astrochron. Additional details are provided in Supporting Information S1.

Results
The major element records primarily reflect terrestrial materials. Detrital silica is the major constituent, consistently about 60% by weight of the sediment. The biogenic contributions are minimal; opal content is low (<4.5%) and wt.%CaCO 3 in the core increases from 2.6% at 63.18 CCSF-A (m) to 4.2% at 1.791 CCSF-A (m). The terrestrial elements decrease across the Holocene with superimposed centennial variability except for Ca, which increases across the Holocene (Figures S6 and S7 in Supporting Information S1). Principal components analysis of the terrestrial elements (Al, Ca, Si, Ti, Fe, K, and Zr) is utilized to isolate common variability within the elemental data and reduce noise (using the "pca" function in Matlab). Element wt% used for the PCA are not on a carbonate-free basis because of minimal biogenic contribution, and low wt.%CaCO 3 sampling frequency. The PCA results group the long-term Holocene trend in Al, Si, Ti, Fe, and K along with the shorter-term variability into the first principal component (PC1), explaining ∼70% of the variability (Tables S2 and S3 in Supporting Information S1). Terrestrial elements have a >0.4 correlation with PC1 except for Ca (0.1), which has a strong correlation (0.7) with PC2.
Terrigenous sediments recovered from the central Chilean Margin are primarily composed of pyroxene, quartz, plagioclase, and mica (Lamy et al., 1998), reflecting the onshore geology. At 44°S, exposed materials are dominated by the calc-alkaline plutonic North Patagonian Batholith, with some intrusive volcanics and metamorphic basement (Adriasola et al., 2006;Davies et al., 2020). Similar long-term trends in Al, Ti, Fe, and K are reflected in a common mode of variability, PC1, which suggests a common origin, likely in pyroxenes. Given very low biogenic %CaCO 3 , Ca association with PC2 indicates a different mineral origin, plausibly Ca-rich plagioclase. Distinct long-term trends of Ca-rich plagioclase and K-rich pyroxene weathering are reasonable given known differences in the resistance of the two minerals to chemical weathering. The Goldich stability series, which ranks mineral weathering susceptibility, indicates that Ca-plagioclase is more easily weatherable than K-rich pyroxene (Goldich, 1938). Decreasing Al, K, Ti, and Fe and increasing Ca are thus consistent with a long-term shift from pyroxene to Ca-plagioclase weathering and a corresponding decrease in weathering intensity across the Holocene (Figure 2).
The Holocene is visibly divided into three distinct periods in the frequency and time domains (Figures 2 and 3). In the early Holocene %Al exhibits a long-term steady decrease from 12 to 5 ka, is relatively stable until 2 ka, then decreases toward the present. MTM spectral analysis of the TSI record identifies statistically significant quasiperiodic variability at timescales of 203, 130 and 88 years (>95% CL), and time-frequency EPSA illustrates amplitude modulation of these terms. The J1004%Al record shows strong statistically significant cyclicity at ∼200 years (>95% CL), significant longer period cycles (830 and 3,527 years), and insignificant cycles at 130 and 80 years (Figure 3 and Figure   Figure 2. J1004 Al wt%, Ca wt%, and K/Ca on a carbonate-free basis, plotted with PC1. Dotted lines are the data set average values and green coloring highlight periods of greatest variability. Popout boxes are enlarged segments of the Al record. 4 of 9 S8 in Supporting Information S1). The mid-Holocene (7-4 ka) is characterized by longer period oscillation, which is absent in the early and late Holocene. In contrast, the 200-year cycle is most powerful in the early and late Holocene.

Terrestrial Input and SWW Strength
Previous studies have suggested that Chilean Margin sediments originate from both Andean and Coastal Range sediments, with higher Ti or Fe counts corresponding to increased Andean contribution and stronger weathering  (Meyers, 2014;Steinhilber et al., 2012). In the power spectra (top), the solid red line represents the Robust AR1 background fit, and the dotted red lines are the 90%, 95%, and 99% confidence levels (Bonferroni-corrected for multiple testing). Background fit for all spectra is justified in Figure S9 in Supporting Information S1. Evolutionary spectra (bottom) show how the signal evolves through time. All analyses use the multitaper method (Thomson, 1982), with three 2π prolate tapers. The Evolutive Power Spectral Analysis utilize a 2,000 years moving window, and power is normalized such that the maximum value in each 2,000 years window is unity. A linear trend was removed prior to all analyses. (Lamy et al., 2001;Siani et al., 2010;Stuut et al., 2007). Aluminum, in contrast, is an erosional product of both source regions which are composed of aluminosilicates and can therefore be considered representative of total lithogenic input (Lamy et al., 1998). Further, its delivery to the margin is unaffected by hydrodynamic processes or grain size (Bertrand et al., 2012). Covariation of Al with K, Ti, and Fe suggests that terrestrial input is dominated by a single region at site J1004 ( Figure S6 in Supporting Information S1). At J1004's location, precipitation decreases when the SWW shift southward ( Figure S10 in Supporting Information S1). We interpret changes in Al wt% to represent changes in weathering intensity, which we attribute to precipitation intensity. Because alkaly pyroxenes are less susceptible to dissolution than alkaline-plagioclase, the concomitant decrease in K/Ca ratio with %Al indicates a decreased weathering intensity with the corollary of a decrease in precipitation intensity as seen by the general match between the two records (Goldich, 1938).
Other studies support our interpretation of J1004. The long term trend in J1004 mirrors migrations of the Intertropical Convergence Zone (ITCZ) with the exception of the bottom 7 m of the core, where a major lithological change suggests a different interpretation of Al values is warranted ( Figures S6 and S11 in Supporting Information S1). XRF data from other Chilean Margin cores show similar trends to J1004, confirming that this is a regional rather than a local signal ( Figure S12 in Supporting Information S1). Records from the Southern Chilean Margin disagree with one another, indicating southward SWW movement on the hyperhumid western side and northward or no movement on the lee-side (Kilian & Lamy, 2012). Modern observations indicate that rainfall totals along the Southern Chilean margin are less sensitive to latitudinal SWW shifts than farther north on the margin ( Figure S10 in Supporting Information S1).
Superimposed on the long-term poleward shift in the SWW across the Holocene, we find that the SWW responded to changes in TSI, as evidenced by coherence with TSI. Studies restricted to the late Holocene find a dominant 200 year period in the SWW, but our record, which extends the high-resolution proxy record, shows that the dominant solar cycle evolves throughout the Holocene along with TSI, and therefore may change again in the future. The lack of long high-resolution records of the SWW has inhibited our understanding of the relationship between SWW and TSI variability, and subsequently our ability to model future SWW variability with confidence.

TSI and Global Climate
Variations in SWW migration cannot directly be caused by the small variations in TSI and must be amplified within the climate system in order to force the observed shifts. Gray et al. (2010) outlined two mechanisms: (a) increased UV radiation directly warms the tropical lower stratosphere and (b) higher TSI in the subtropics increases evaporation and transports water equatorward. Combined, these processes increase (decrease) the latitudinal temperature gradient and lead to Hadley Cell expansion (contraction) and southward (northward) ITCZ migration when TSI increases (decreases). Short term ITCZ migrations during decadal solar cycles have been detected by modern observations, whereas 200 and 500-year cycles have been observed in paleoclimate records (Gusev & Martin, 2012;Poore et al., 2004). Our examination of a Peruvian stalagmite record with <5-year resolution that reflects increased precipitation with ITCZ southward displacement, found signals at these periods above background level, however they are statistically insignificant following application of the Bonferroni multiple-testing correction (van Breukelen et al., 2008) (Figure S13 in Supporting Information S1). We find similar results from other records expected to be influenced by this process.
Late Holocene SWW data sets have identified periods matching solar cycles and paired models connect the signal to a climate response. Varma et al. (2011) concluded that based on proxy and model data, TSI has significantly influenced SWW position for at least the last 3,000 years. By artificially amplifying TSI variability, Wright et al. (2022) show that modeled SWW position better aligns with proxy data than previous model simulations that do not amplify the solar signal and that TSI influence on SWW position exceeded the range of internal variability. Both the Varma and Wright models agree that as TSI increases, the SWW move southward and vice versa. Our data present indirect, though compelling evidence supporting solar influence on the SWW during the entire Holocene.

Antarctic Connection
Previous studies have demonstrated a link between SWW position and Antarctic upwelling on millennial timescales. It is therefore plausible that SWW movement during solar cycles could generate a high latitude climate response Russell et al., 2006). Our analysis of Antarctic temperature data demonstrates a signal at a 200 year period significant at the 95% confidence level, though not at the more conservative Bonferonni-corrected 95% level (Jouzel et al., 2007) (Figure S13 and Table S4 in Supporting Information S1). Antarctic temperatures have experienced a disproportionate response to small direct solar forcing variability, with substantial cooling occurring during periods of increasing TSI (Mayewski et al., 2005;Zhao & Feng, 2015). Swingedouw et al. (2010) propose that Antarctic cooling in response to increased solar forcing in model simulations of solar cycles could be the result of SWW southward movement and increased Southern Ocean cold water upwelling. Our results enable us to expand on this idea and propose a mechanism wherein the SWW serve as an intermediary imprinting amplified TSI periodicity on Antarctic temperature ( Figure 4). First, increased TSI expands the Hadley Cell and moves the SWW poleward, corresponding with a southward Subpolar front shift and stronger PDW flow. At the J1004 site water depth on the boundary of PDW and AAIW, when PDW is stronger (weaker) associated with a southward shift of SWW, we might expect more negative (positive) δ 13 C values from PDW expansion (shoaling). Benthic carbon isotopes show similar multi-centennial scale variability to terrestrial XRF data with higher (lower) Al wt% concomitant with more positive (negative) δ 13 C values, supporting solar influence on deep water circulation and stronger PDW flow when the SWW shift south ( Figure 5). Benthic δ 13 C values experience large variability on the order of ∼200 years in the early and late Holocene and longer cycles in the mid Holocene, similar to what is observed in the Al wt%. Further south, PDW outcrops within the ACC bringing cold deep water to the relatively warm surface, thus sequestering heat and cooling the air temperatures when TSI increases (Figures S4 and S14 in Supporting Information S1) (Lovenduski, 2005;Swart et al., 2019).

Future Implications
Centennial scale solar cycles influence the climate system through atmospheric circulation changes in the tropics and extratropics. We find periodicity in the SWW matching known solar cycles throughout the Holocene, with appearance and disappearance of the 200 years cycle consistent with the TSI record. Additionally, we provide evidence that the SWW transmit and amplify the TSI perturbation from the tropics into the high latitudes, impacting Southern Ocean upwelling and Antarctic temperatures.
In the present, TSI is relatively low but anthropogenic factors have been forcing SWW movement southward since the 1970s (Abram et al., 2014). As a result, Southern Hemisphere populations have faced dry conditions causing water scarcity and wildfires (Garreaud et al., 2009;Holz et al., 2017;Iglesias et al., 2016;Schneider & Gies, 2004;Veblen et al., 1999). At the time of writing, Chile is in its 13th year of drought (Bartlett, 2022;Garreaud et al., 2020). Furthermore, by the end of the current century, predictions suggest TSI will increase again (Steinhilber & Beer, 2013), with the potential to exacerbate this southward drive. With greater understanding of natural variability, populations living in the effected region can better prepare for periods of extreme, extended . Proposed mechanism for amplified total solar irradiance (TSI) signal transfer to Antarctic temperature. Increased TSI is amplified in the tropics and subtropics by the top-down and bottom-up approaches. This expands the Hadley Cell, shifting the Southern Westerly Winds southwards. Changes to atmospheric circulation also shift oceanic fronts, increasing cold water upwelling to the surface within the Antarctic Circumpolar Current resulting in atmospheric heat loss.

Figure 5.
A comparison of detrended J1004 terrestrial X-ray Fluorescence data represented by Al wt%, J1004 benthic δ 13 C from species Uvigerina peregrina, and total solar irradiance (TSI) (Steinhilber et al., 2012). A 40-point moving average is applied to J1004 and a 4-point moving average is applied to TSI to have a consistent temporal smoothing for both records and to remove noise.